How to design a city?

As opposed to completely unsubstantiated opinions with "FACT" written after them?

Congestion is a poor word to use, what is important is maximizing the flow rate.

Traffic does flow rate in increased better with slower traffic. Study some models of traffic flow, validated by reams of data. Of course it depends on various factors and the fact that it is an urban environment will have an affect. Just because something is non intuitive does not mean it's untrue.

Unrelated to flow rates above but lets consider a simplified traffic system.

One point on congestion which is some what of a paradox is that if someone is doing a journey by car through a city and the congestion has increased the journey time by 100% over a journey without congestion but still obeying all speed limits plays into the hands of people who do short unnecessary journeys by car. Precisely the type of journey.

Consider person A. Person A is traveling right across town, say with a load so public transport is not a option and the journey will 60 min in a uncongested city may now take 120 min in congestion. This will be a big and expensive hassel for person A in terms of time and money but they have no option.

Consider person B. Person B is also traveling right across town, this time without a load so public transport is a option. The journey will 60 min in a uncongested city may now take 120 min in congestion. Person B may consider taking public transport depending on the time gain and weather or not they have any stops to make on the way. Simplifying this letts say 50% of the time person B takes public transport 50% of the time they drive.

Now consider person C. Person C traveling 10 minuets down the road, congestion means this journey will now take 20 min. Again relatively 100% increase in journey time. Relative increase however do not take into consideration human psychology. For them the relative increase in journey time is not important, it's only 10 min extra. Public transport may be quicker but it's always nicer in a car and at the end of the day, it's only ten minuets!

Now lets again make the simplification that on city roads the types of journey are split equally into three part consisting of people of each time of person. I know a massive simplification but I believe it serves to
show the basic idea.

So one third of the traffic is there to stay (people of type A), one third could easily swap (people of type C) but are unlikely to as it's only 10 minuets! and half of people of type B stay on the road.

So now we have a total number of people of 1/3+1/6+1/3=5/6 of the the previous amount of traffic. Unfortunately the roads will refill up to the same maximum level but of new proportions. Of this new make up of traffic 3/5 are actually journeys which could be done on public transport. Now this is less than when this though experiment started (it was 2/3 then) but the only type of people to move onto public transport are half of type B people. We can keep iterating through this process. So the lower limit becomes 50% of journey in our idealized city roads could be done by public transport.

Hence the people for who congestion is the least problem cause 1/2 of the congestion and hence why getting people out of cars for short unnecessary journey will always be the most difficult.

Beyond a specific population diffusion / density ratio congestion is inevitable.

my point about 20 mph is mainly health and safety. cyclists and pedestrians tend to be less hurt if the motor powered vehicle (mpv) is going slower.

but yes cars at 20 mph are less noisy - fact. there are govn noise maps and surveys that have already established that.

reducing congestion. yes probably. what has happened in this country is that as the govn has built roads to reduce congestion people have just travelled further taking more overall road space. so over a couple of generations people have gone from say 15000 miles a year lifestyles to say 100000+ miles a year lifestyles. and pollution levels have gone up commensurately.

it has taken us 50 years to get to this car is king culture lifestyle and i reckon it may take us another 50 years to get away from it (to avoid social unrest).

what hmg need to do is break this vicious cycle. by making a bold statement that one can have a safe, good and enjoyable lifestyle by travelling less. and i think that 20 mph is the way to do it. and once that happens people will start making decisions that they can live a life without a car.

and then you start to stop criminalisation of the poor. many people cannot afford a car but because they wanna participate in the car culture so they drive illegally. and yes they add to congestion and kill and maim and pollute and hit and run etc. met police reckon that up to a third of london drivers are illegal. so if you break the car culture then hopefully you take away their reason to break the car laws. sure they may break other laws but hopefully not driving into me when i riding my bike.

i dont believe a 20 mph is ridiculous at all. i reckon it would be well good. and not just for cities, but for towns and villages as well.

i hope that is helpful.

my point about 20 mph is mainly health and safety. cyclists and pedestrians tend to be less hurt if the motor powered vehicle (mpv) is going slower.

Yes, but where do you draw the line? Why not make it 15mph for safety reasons? Or 10mph?

what hmg need to do is break this vicious cycle. by making a bold statement that one can have a safe, good and enjoyable lifestyle by travelling less. and i think that 20 mph is the way to do it. and once that happens people will start making decisions that they can live a life without a car.

I don't think making the car worse is an option, there are already so many people who have an extreme dislike for the governments decisions, they have do something positive, like improve public transport.

Yes, but where do you draw the line? Why not make it 15mph for safety reasons? Or 10mph?
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To be fair most Euorpean countries have 30 kph speed limits in most built up areas which is <20mph so it would bring us in line with them.

To be fair most Euorpean countries have 30 kph speed limits in most built up areas which is <20mph so it would bring us in line with them.

Sure, built up areas, fine, schools, fine, not a whole city though!

Speed does not cause accidents, it only determines the magnitude of an collision after it has occured the majority of accidents are caused by dangerous driving, because you can't quantify dangerous driving, and you can quantify speed, it's much easier to say that speed causes accidents, as that way it is easier to "solve" the problem.

The real solution is to improve driver skills and reactions.

@mmccarthy

The 20 mph is not really the entire city from my understanding. It's not going to include the A4 for example or many other other trunk roads but these are few and far between. Even the south circular, a main trunk road would not be affected by running a 20 mph limit on it. I don't have a figure but I would guess well over 90% of London's roads are not big dual carriage ways that would be affected by running a 20 mph zone. Take the A13 west of the Blackwall tunnel. Major road but if you drive along it even at night there are very few instance wher you go faster than 20 mph for more than a few seconds. If you actually drive at 20 mph you end up going through the same set of traffic lights at the same time due to the traffic flow, but you and your passenger have a much smoother ride. As I said I don't think anyone is seriously saying put 20 mph on every single road like the A12 or the A13 east of Blackwall or the A40 e.t.c but over 90+% other routes would not experience an increase in journey times and hence a 20 mph speed limit would increase safety with no bad side. The perception of going fast by getting upto a higher top speed in between traffic lights is very different to the reality of an increase average speed. You keep on that you are a engineering student so I would expect you to have not problems with this concept and I would expect at some point in your studies you may have the opportunity to look at some traffic flow models.

Re: Speed does not cause accidents.

Of course you are correct that it does not necessarily (it's excessive speed) cause accidents and driver skills being improved but remember

1. We are talking about an urban environment here where there may be the opportunity to go faster for a short time perfectly safely however due to the the fact we are dealing with a urban environment this will be a very short distance. The result will mean that to continue to progress safely a reduction in speed will soon be needed.
   
   

20 kmph = 20,000 mph= 5.55... m/s approx 6 m/s

100 kmph= 27.777 m/s approx 28 m/s

Lets take the example that we are driving along a road at 20 kmph or 6 m/s. Ahead there is a 200 meter section where it is perfectly safe to do 100 kmph (29 m/s). This is quite a decent stretch in an urban environment and 100 kmph is would be very fast.

Now letts assume for simplicity that the car has instant acceleration and deceleration hence as soon as it is in the "fast zone it is doing 100 kmph and back down to 20km at the end. If the car chooses to drive at 100 kmph for the fast section it will complete the section in (200/28) seconds = 7.14 sec.

Now consider the car carries on at 20 kmph the same stretch will have taken (200/6) seconds = 33.333 seconds. A 26.19 seconds difference.

The car traveling fast with there for be 26.19*6=157.14 meters further down the road than the car which continued along at 20 kmph.

Sounds good on the face of it but this model is extremely favorable toward the fast car as it assumes instant acceleration and deceleration, a very high top speed over quite a long stretch plus there are no junction / traffic lights to stop the fast car allowing the slow to catch up some. None the less we shall keep with this.

Now lets assume this opportunity happens once ever 2 km. So over a 10 km journey there will be five opportunities for this resulting in a approx 26.19*5 second = 130 seconds advantage over a route that would take the slow car 1,666 sec journey, so a 7.8% saving in time. now as I said this is swinging every thing in favor of the very fast car and the saving is still only 7.8%. In reality the saving would be less than 2% I suspect if I could be bothered to model it more accurately. Again remember this is in a urban environment so journeys will be quite short so even at nearly 8% the time saved will be minimal. For burning alot of extra fuel.

1. Human self control it not it's greatest. Even if you allow higher speed limits in many areas as in the example above (or just keep 30 mph everywhere as it is now) the chance that people will slow done enough during the time when they are meant to slow down is minimal. You may do, but most will not. no matter how good the training you must remember most people are very stupid and very selfish ( would include my self in this too).

2. The law always a blunt instrument. It has to work on the lowest common denominator.
   For example I regularly cycle through red lights, but I don't believe the law should be that you should be free to cycle though red lights because the law would have to be so complicated and long with all the if this if that then it's okay to jump a red light that it would become useless. Much the same with speeding in general and most defiantly in an urban environment this is the best tool that is avalible to us whit out resorting ot extreme surveillance.

You keep on that you are a engineering student

I do apologise if anyone got the feeling that i'm trying to say my opinion is more valid than anyone elses by stating that, I don't mean to be patronising, I, like mst of the others here am just trying to think of ways to improve the situation.

Completely agree with you on point 1

No 100% with you on point 2, yes, there are a (worringly) large number of people who believe they are driving safely if they are under the speed limit, it should be common sense that you slow down in densely populated areas such as schools, unfortunately this doesn't apply to everyone, once again, drivers should be more educated in these matters.

"The law always a blunt instrument. It has to work on the lowest common denominator."

Unfortunately you are 100% right on that, and I know that it would be extremely difficult to punish people on the grounds of dangerous driving compared with speeding, it's a shame, like I said above, either improve driver education dramatically (which will be difficult) or make the public transport system so much better that the only people who will continue to drive are those that are enthusiastic about it (i'm gonna get flamed for this next part) and who appreciate driving as an art, and want to do it well.

Speed does not cause accidents, it only determines the magnitude of an collision after it has occured the majority of accidents are caused by dangerous driving, because you can't quantify dangerous driving, and you can quantify speed, it's much easier to say that speed causes accidents, as that way it is easier to "solve" the problem.

The real solution is to improve driver skills and reactions.

Michael, this is definitely not true. Speed is most certainly a major contributory factor in many (though of course not all) collisions and this is backed up by copious research, e.g. a much-misquoted TRL report.

It is really not a solution to just improve driver skills and reactions. It needs to be at least coupled with a commitment to drive slowly. High skill levels among drivers, paradoxically, can cause higher speeds. What matters is the degree of commitment that they have to driving in such a way that does not cause fear and road danger. Plus, some highly skilled drivers, e.g. racing drivers driving in normal traffic, exercising their skills can scare the living daylights out of other, less skilled drivers.

The key is always, always, to reduce road danger at source. That means avoiding high speeds in the first place, not attempting to manage them using second-best sticking plasters.

Someone hit by a car doing 20mph has a 95% chance of survival and only a 55% chance at 30mph. The vast majority of those who survive a 30mph crash will sustain serious injuries.

Also beware of the word 'accident'. It doesn't do justice to the fact that rates of collision are almost built-in system features rather than accidental. Collision rates have gone down gradually but often at the expense of people shying away from cycling and walking and using some sophisticated statistical comparisons haven't improved much in the last fifteen years.

I do apologise if anyone got the feeling that i'm trying to say my opinion is more valid than anyone elses by stating that, I don't mean to be patronising, I, like mst of the others here am just trying to think of ways to improve the situation.

Sorry not meant to be a dig, more a compliment to the fact that you should have a pretty good analytical mind, a good grasp of maths e.t.c. A very important skill I.M.O (and it should be given my world). Bad choice of sentence structure on my part. This is quite an interesting thread so keep putting across your opinion, I'm not trying to bullzoe either, even if I seem like I am.

EDIT: P.S The traffic flow modeling is serious. If you have an opportunity to do a general mathematical modeling course that may have traffic flow modeling in it. I did maths at uni and took a course on this which had some mathematicians and some engineers, but as ever it depends what is available to you. It's quite interesting and the basics models are very quickly taught.

Michael, this is definitely not true. Speed is most certainly a major contributory factor in many (though of course not all) collisions and this is backed up by copious research, e.g. a much-misquoted TRL report.

Yes, I agree, what I was trying to say was that travelling 70mph on a motorway should be perfectly safe, but travelling at 70mph in a 30mph zone is normally unsafe (I say normally as there are some ridiculous open stretches of road around with a silly 30mph limit). Motorways are the safest roads of all, and basically the lower the speed limits, the higher risk on a road, I know that sounds obvious, but I can't stand it when all this research is done and they put "Speed Kills" at the top in huge letters. Yes, I know there are more oppurtunities for collisions on 30mph roads than motorways, but I just wanted to make that point against the "Speed Kills" believers.

It is really not a solution to just improve driver skills and reactions. It needs to be at least coupled with a commitment to drive slowly. High skill levels among drivers, paradoxically, can cause higher speeds. What matters is the degree of commitment that they have to driving in such a way that does not cause fear and road danger. Plus, some highly skilled drivers, e.g. racing drivers driving in normal traffic, exercising their skills can scare the living daylights out of other, less skilled drivers.

I do disagree with this a little, in my experience (obviously I can't speak for all) but most high skilled drivers are very safe on normal roads as they observational skills (in my mind, probably the most important skill in a driver) are excellent, same goes for racing drivers, it would take a very irresponsible racing driver to push their limits in public traffic. When I said about improving driver skills, I wasn't talking about cornering skills or things like that, I was talking about (as above) observational skills, reaction times (i'm sure you all know the kind of person who half pulls out of a side-road, wait until traffic is almost on top of them, then pulls out of the side road, very frustrating).

The key is always, always, to reduce road danger at source. That means avoiding high speeds in the first place, not attempting to manage them using second-best sticking plasters.

Someone hit by a car doing 20mph has a 95% chance of survival and only a 55% chance at 30mph. The vast majority of those who survive a 30mph crash will sustain serious injuries.

Also beware of the word 'accident'. It doesn't do justice to the fact that rates of collision are almost built-in system features rather than accidental. Collision rates have gone down gradually but often at the expense of people shying away from cycling and walking and using some sophisticated statistical comparisons haven't improved much in the last fifteen years.

This may seem a bit pedantic of me, but high speeds are not a problem, excessively high speeds usually are, take my example in the first paragraph. I'd say that survival statistic is pretty accurate (i'm guessing it's for car/ped collisions) however, there is no situation where cars and pedestrians should be on the same tarmac, this may sound ignorant of me but I don't mean it like that, but I tend to not pay (relatively speaking) a huge amount of attention to those. True, I am still in the habit of using accident instead of collision (have to watch Hot Fuzz a few more times). In short, I try not to pay much attention to statistics unless I have got the data myself, as it will have no doubt been twisted in some shape or form between collection and presentation.

Apologies for the long reply.

Sorry not meant to be a dig, more a compliment to the fact that you should have a pretty good analytical mind, a good grasp of maths e.t.c. A very important skill I.M.O (and it should be given my world). Bad choice of sentence structure on my part. This is quite an interesting thread so keep putting across your opinion, I'm not trying to bullzoe either, even if I seem like I am.

EDIT: P.S The traffic flow modeling is serious. If you have an opportunity to do a general mathematical modeling course that may have traffic flow modeling in it. I did maths at uni and took a course on this which had some mathematicians and some engineers, but as ever it depends what is available to you. It's quite interesting and the basics models are very quickly taught.

No worries, worst thing about forums is you can't hear the other person's tone of voice so you sometimes get it mixed up what they're trying to say.

That traffic modelling module does sound pretty interesting, unfortunately i'm pretty sure it's not an option on my course though, seems like it may be more of a Civil Engineering aspect.

Hello BlueQuinn,

sorry for slow replies, I've only just got back to London and have been too busy to post at length. (Sighs of relief everywhere I'm sure.)

Surely you would define congestion as lots of closely packed cars driving slowly.

Actually, the most inefficient aspect of congestion is uneven speeds, i.e. cars fully stationary one moment and burst speeding the next.

Ergo, slowing cars down, perhaps by means of a 20mph limit will make congestion worse.
You forget driver psychology. Drivers are strongly attracted to the illusion of fast travel in cars. (NB we're talking about traffic in towns, of course, not on rural roads or other relatively uncongested environments.) Take that away and the trips that we're predominantly initially targeting to change travel behaviour, i.e. 1-2 miles, no loads, no passengers, driven by people without mobility difficulties, suddenly come to be perceived as slow by car.

I doubt that traffic flow would improve. You'd still have the same stop-start burst speed thing as now, just slower longer-lasting.
No, actually. The problem is that queues form very quickly if high top speeds are allowed, much more quickly than they can move off at the front. The ratio of the speeds at which queues form to the speed at which they move off at the front is much reduced with lower top speeds.

Try imagine someone 'burst speeding' at 20mph. Not very scary, is it?

And emissions worse, as (thanks to the speeds the official fuel consumption figures are measured at) cars are not designed to be efficient at 20mph, but at 31 and 56. Car design has to change to make 20mph cleaner (but all that takes is for european union/member states to also demand an official fuel consumption reading at 20mph)
Very true. Optimising cars' fuel use at 20mph or lower is a very important area of work facing the engineers of the present and the future, perhaps something Michael will be able to work on when it has filtered through to motorsport, too. ;)

DaveH said "slower cars = increased flow of traffic as you can fit more cars on the road"
This is inaccurate or confused.

Slower cars, by definition, is actually decreasing the flow of traffic. It is increasing the density of traffic, which is a different thing. Increased traffic flow is defined as cars moving faster.
'Flow' is typically used to indicate a volume/number of cars (e.g., '5 cars a minute were observed travelling eastbound') rather than the speed of a flow. Small matter of terminology, I think.

To eliminate or severely reduce the use of cars in cities the most effective measure is probably to eliminate parking facilities. We all want nice roads to ride on, and buses need them too. If we ban all city centre developments from providing car parking spaces, and remove all car parks and street parking, then people won't drive into the centre of town simply because there is no point. I suppose we could actually just ban cars altogether, but given that you need people to have traffic sense so that police, ambulance, buses, taxis etc can still operate safely it's probably a good idea to allow cars and just rely on the fact that it will only be through traffic to massively reduce car use.
This is true. Reducing easy availability of car parking is an important tool in the toolkit and this has long been recognised by planners. There are quite a few positives on it in the London Plan, especially for city centre office developments. Not nearly enough yet. Many London boroughs still have planning policies that specify x number of car parking spaces per dwelling unit and there's an inertia in the planning system that's very hard to speed up.

It is interesting how public transport is being touted as the solution, but really, how many of us use it unless we are forced to? We cycle in all weathers because tubes and buses are filthy, slow, dangerous, expensive and generally horrible. You can't eliminate any of those factors.
I never understand what people's problem is with the fact that public transport is used by, erm, the public. I don't use it much, as I prefer to cycle, but I've just had a very pleasant and effective bus journey back from the train station, using first the 476 and then the 106, admittedly late at night, but fast, clean buses with good drivers and excellent frequency of service. I have to say that London's bus system (I think the last time I used the Underground must have been about seven years ago) is starting to feel more and more Continental in quality compared to what it used to be.

Still, I can't say anything about habitual use. The main reason why I prefer cycling (apart from all the other attendant benefits like fitness) is because it's a detour-free mode (unless I want to make a detour) and it's more efficient for the vast majority of my trips than public transport.

Public transport is slow because it doesn't start and finish where you want it to and it keeps stopping to let other people on. It is dirty and dangerous because it lets the public on, who leave it in a state, spread their germs around, fail to wash, can be drunk, violent, homicidal etc. (and that's just the drivers, boom boom!).

I'm afraid this sounds seriously sociopathic to me. Are you sure you didn't mean the whole bit tongue-in-cheek rather than just the bit about drivers?

Ergo public transport will always be a less desirable option to those who drive a car, no matter how much you spend on it.
This is not true. Again, think not so much in terms of monomodality for users, but in terms of individual trips. Greater investment in public transport will gradually make certain trips more attractive to certain people. They won't get rid of their cars altogether but use them less. Now, I'm no major public transport advocate, as I think (unsurprisingly) that cycling should be given a much greater role (and p/t will of course never achieve 100% coverage of possible trips, nor should it), but you will find that a lot of people don't mind using p/t at all

I don't think you can price people out of their cars. you have to find other ways, and removing their practicality is probably the best option.
Well, obviously you can price people out of their cars, but it takes a more joined-up approach than just congestion charging, and also dynamic charges that respond to economic conditions. Needless to say, this is something that no politician would risk, as it would be perceived as predominantly negative. You're absolutely right that reducing their practicality is important, but at the same time we have to be quite clear that even right now for a lot of journeys cars have very little transportational utility.

I'll respond to your subsidy post when I get a minute later.

EDIT: P.S The traffic flow modeling is serious. If you have an opportunity to do a general mathematical modeling course that may have traffic flow modeling in it. I did maths at uni and took a course on this which had some mathematicians and some engineers, but as ever it depends what is available to you. It's quite interesting and the basics models are very quickly taught.

True, but as I'm sure you're aware traffic modelling also bears a few hazards, as often the models are heavily simplified and streamlined to enable engineers to show that schemes fulfil something like an 85th percentile capacity peak hour requirement, as part of whatever scheme justification they have to do.

They are also only as good as what is put in, i.e. if no motor traffic reduction or a projected increase is factored in to start with they will do a huge disservice, as they often lead to business as usual and often to increased motor traffic capacity. We are still very far from sophisticated models being applied generally. It is also quite hard, perhaps impossible, to model cyclists and pedestrians.

True, but as I'm sure you're aware traffic modelling also bears a few hazards, as often the models are heavily simplified and streamlined to enable engineers to show that schemes fulfil something like an 85th percentile capacity peak hour requirement, as part of whatever scheme justification they have to do.

They are also only as good as what is put in, i.e. if no motor traffic reduction or a projected increase is factored in to start with they will do a huge disservice, as they often lead to business as usual and often to increased motor traffic capacity. We are still very far from sophisticated models being applied generally. It is also quite hard, perhaps impossible, to model cyclists and pedestrians.

Unfortunately you have clearly been given modeling data in a incorrect light. The point of models frequently is not in general to get out a number. It's to ascertain what are the general mechanisms behind a physical problem are, what the relationship is between factor and how strongly they are linked. It should also be fully stated in what regions / conditions the model is valid. As far as traffic modeling goes you would have to have several models for different situations, much like every other physical situations.

I expect most government models are using very old methods, but employing some decent mathematicians / engineers to do the job is prob not high on the department for transports list. Many government departments just have some program that they bought and no qualified people to a actually model.

As far as modeling cyclists and pedestrians into a traffic flow I see little point how do you mean? If you mean their individual flow and how street planning allows the flow of ped / bikes, in which case that should be easy. Or do you mean adding ped / bike flow in with vehicular flow? If so there is little point as the same effect can be achieved in other ways which I ma willing ot discuss at a pub with pen and paper.

Hi Oliver,

Yes, I was stating an extreme, tongue in cheek statement of why public transport is crap, but each of those objections is true enough to be used by a car driver (and me on my bike) to justify their continued avoidance of public transport. Humans are not really social animals (or else vibrant conversations would be the norm in lifts), so being forced to share space with others is stressful. For many it is more stressful than being stuck in a traffic jam, or coping with the risks and dangers of cycling on London's streets. (And I think driving a car is already far more difficult and expensive than getting on a bus, so I think we may be well into a law of diminishing returns vis a vis pricing drivers off the road.)

My problem with the advocacy of 20mph limits as any kind of solution is that you assume people will obey it. I seriously doubt that they will. On my commute, (along what I would consider major roads for the most part, though not all are red routes) I would say that in the average London street with a 30 limit, when they can drive freely the modal traffic speed regularly reaches 40-50mph. In a 20 limit such as some in Islington (sorry really bad at street names), where the road is a nice wide main route and the limit is stupid then the max traffic speed is 30-35. Drivers will drive at a speed they perceive to be safe for the road and conditions (which may well be too high of course), and will ignore speed limits. I just don't think it will work (blanket fixes rarely do). A more creative solution is needed.

I personally don't like (current) 20 limits because they are designed to be self enforcing. This means road humps, chicanes and other hazards which are just not cycle-friendly, even when they have supposedly been designed to be. gutters fill up with broken glass and stuff. Split humps force you into the path of cars, etc. Static speed cameras are useless because they only enforce speed at one point, which, like traffic calming measures, directly contributes to the "slam on the brakes, then speed up again" practice. And sometimes this itself causes accidents. Average speed cameras can't work on any road with junctions, crossings, bus stops, street parking, traffic lights or any other interruptions: they can only be effective in a decent stretch of uninterrupted road - e.g a motorway - which is precisely where speed enforcement is least needed.

Also, this is a personal thing, but I don't find queues of slow moving or stationary traffic that nice to cycle near. There are too many fumes, cars get in your way, etc. Moving traffic is nicer, because cars just overtake you and then they are out of your hair.

what i think we need is a gradual approach to reducing mpv dependency. and yes through a range of measures but yes definitely to include community safety - 20 mph.

but also adjusting taxes for example. maybe reducing income tax and increasing pollution taxes. and rewarding those who make the shift by cutting their overall tax burden.

sure blue quinn we need to get creative and use loads of tools to change behaviour. and city planning is just one tool.

it can be done and i think it will have to be done. but possibly due to scarce resources (fossil fuels running out) more than global warming or community safety.

Unfortunately you have clearly been given modeling data in a incorrect light. The point of models frequently is not in general to get out a number.

Hm, I'm not quite sure what in my post gave you the impression that I think that, perhaps the 85th percentile remark. That only referred to a traffic engineering requirement to have traffic environments functioning at that sort of level of saturation. The model itself of course contains the actual parameters of the (proposed) scheme.

The criticism I meant to articulate is aimed largely at the requirement to have the model tested with at least existing motor traffic volumes, rather than assuming a reduction in motor traffic volume, which often vitiates any kind of innovative solution, and you get the usual nonsense that's designed to manage road danger rather than reduce it at source--guard railings, pig pen dog leg crossings, dual carriageways, central hatching, etc.

As far as traffic modeling goes you would have to have several models for different situations, much like every other physical situations.

Of course, for different scheme options, for instance.

I expect most government models are using very old methods, but employing some decent mathematicians / engineers to do the job is prob not high on the department for transports list. Many government departments just have some program that they bought and no qualified people to a actually model.

There are some very old programmes around, largely looked after and updated by people like TRL.

As far as modeling cyclists and pedestrians into a traffic flow I see little point how do you mean? If you mean their individual flow and how street planning allows the flow of ped / bikes, in which case that should be easy. Or do you mean adding ped / bike flow in with vehicular flow? If so there is little point as the same effect can be achieved in other ways which I ma willing ot discuss at a pub with pen and paper.

Walking and cycling are not mobility modes, which means that they are less modellable than motors. For instance, one cyclist may ride in the gutter in an intimidated and victimised way, whereas another may take the primary position and not be overtakeable. Pedestrians may step out into the street randomly and cause vehicular flow to stop. It's not programming this so that it occurs in a model that's the problem, but actually getting a handle on how to quantify it/what is likely to happen. It can seriously affect the accuracy of a model. It's one reason why cycling is often a mere afterthought in the design process and why we're generally not even getting cycle audits after schemes that didn't consider cycling had been designed.

Hi BlueQuinn,

Yes, I was stating an extreme, tongue in cheek statement of why public transport is crap, but each of those objections is true enough to be used by a car driver (and me on my bike) to justify their continued avoidance of public transport.

I don't agree that they're true, though. I think they're exaggerated fears. That doesn't change that people have these fears, of course. But I think you'll find that one of the surest indicators that people aren't using the mode of their free choice is when they are down on other people doing the same thing. It's actually a well-attested phenomenon in people who drive a lot, whether through choice or because they have to.

Humans are not really social animals (or else vibrant conversations would be the norm in lifts), so being forced to share space with others is stressful.

Now, this is transparently not true, either. We are most definitely social animals, but there obviously comes a point where the number of people around becomes completely unmanageable. I often notice when people come to visit me from smaller places that they are used to interacting with people in a completely different way than an experienced Londoner. So, while we are social, we are of course not so social that mere air is the social glue that binds us. If you said 'being forced to share space with unknown others with whom we cannot develop a meaningful and positive relationship for whatever reason, is stressful' I'd agree. But not to the broader statement above.

And as to the origin of the expression, consider that Aristotle actually said 'politikon zoon' when he said that. Politikon doesn't just simply mean 'social'. It is cognate with polis, a community or body of citizens, or, at the height of classical Greek culture, a city-state. Nowadays there is a much stronger separation of 'the social' and 'the political', perhaps partly because there are just so many people and place counts for so little. Internet forums are just one of the bizarre blossoms of this.

My problem with the advocacy of 20mph limits as any kind of solution is that you assume people will obey it. I seriously doubt that they will. On my commute, (along what I would consider major roads for the most part, though not all are red routes) I would say that in the average London street with a 30 limit, when they can drive freely the modal traffic speed regularly reaches 40-50mph. In a 20 limit such as some in Islington (sorry really bad at street names), where the road is a nice wide main route and the limit is stupid then the max traffic speed is 30-35. Drivers will drive at a speed they perceive to be safe for the road and conditions (which may well be too high of course), and will ignore speed limits. I just don't think it will work (blanket fixes rarely do). A more creative solution is needed.

I think you partly give the answer you need yourself. I don't have to assume that people will obey 20mph--that it will reduce speeds and casualties has been proven beyond any reasonable doubt in oodles of studies, as a quick Google will confirm. Yes, drivers will exceed speed limits, but they won't ignore them entirely, as they will drive at what they consider to be a defensible (in their view) degree to which they exceed the limit. 20mph works.

I personally don't like (current) 20 limits because they are designed to be self enforcing. This means road humps, chicanes and other hazards which are just not cycle-friendly, even when they have supposedly been designed to be. gutters fill up with broken glass and stuff. Split humps force you into the path of cars, etc.

This is all true, of course and I quite agree. I myself am particularly annoyed that the concept of 'road' or 'street' (ideally flat) is led ad absurdum by the addition of all this stuff. The problem is that again, these things are done because they demonstrably reduce casualties. The rule of thumb used to be by 40% where humps or tables (full width) are employed, and 20% where cushions (the ones you call 'split humps') are used. I suspect that with advances in car suspension probably partly triggered by just these vertical deflection measures, the percentages must now be lower. Still, traffic authorities are still required to produce annual reviews and three-year monitoring of scheme outcomes, and I haven't heard anything about these casualty reductions not being reached by safety schemes any more.

Static speed cameras are useless because they only enforce speed at one point, which, like traffic calming measures, directly contributes to the "slam on the brakes, then speed up again" practice. And sometimes this itself causes accidents.

Well, as ever, (chorus) 'if people weren't breaking the speed limit in the first instance, they wouldn't have to slam on the brakes' (/chorus). :) As far as I'm aware, the 'accident' argument is an urban or rural myth one dreamed up by the anti-speed camera lobby. It is also downright bizarre. (I do note that you say 'sometimes' are not perpetuating the myth as it's often stated.)

The toolkit is expanding, of course, but as you said initially, the problem is that much of it is supposed to be self-enforcing. That's because there's hardly any traffic police in this country. I mentioned this to a German police person recently. His jaw would have hit Australia had there not been a planet in the way.

Average speed cameras can't work on any road with junctions, crossings, bus stops, street parking, traffic lights or any other interruptions: they can only be effective in a decent stretch of uninterrupted road - e.g a motorway - which is precisely where speed enforcement is least needed.

You'd be surprised how many locations there are where average speed cameras can work just fine! They can definitely stop most of the rat-running even if they can't catch every plonker who does 50 for a short stretch and then gets stuck somehow, lowering his average. It isn't correct to exclude all of the location features you list, as if despite these obstacles someone gets through an area, say a 20mph zone, at a faster average than is possible at 20mph, then they trigger the enforcement mechanism.

Also, this is a personal thing, but I don't find queues of slow moving or stationary traffic that nice to cycle near. There are too many fumes, cars get in your way, etc. Moving traffic is nicer, because cars just overtake you and then they are out of your hair.

For sure.

Now I've run out of time again to reply to you on subsidies, coming soon, promise. :)

Yes, drivers will exceed speed limits, but they won't ignore them entirely, as they will drive at what they consider to be a defensible (in their view) degree to which they exceed the limit. 20mph works.

But if the aim of the 20 limit is to slow them down to 30, why risk criminalising people? Why not just enforce the 30 limit?

The problem is that again, these things are done because they demonstrably reduce casualties.

I read that the London Ambulance Service wanted them all removed because by reducing response times, forcing ambulances to avoid certain roads, etc, they cost more lives than they save. If that is true there is no justification for them at all.

I suspect that with advances in car suspension probably partly triggered by just these vertical deflection measures, the percentages must now be lower.

I'm not aware that there have been any real advances in car suspension since the 1960s. normal cars get wrecked by speed humps. Which, as a result, means that speed bumps are directly responsible for the massive increase in big off-roaders in cities, because of their ground clearance. I don't know if that genie could be put back in the bottle by removing humps, but putting them there definitely killed the small city car in favour of the SUV.

Well, as ever, (chorus) 'if people weren't breaking the speed limit in the first instance, they wouldn't have to slam on the brakes' (/chorus). :)

You still get this when obeying the speed limit. Have you ever tried going down a road with speed humps at a constant 20mph? It is impossible. You have to brake then accelerate.

As far as I'm aware, the 'accident' argument is an urban or rural myth one dreamed up by the anti-speed camera lobby.

I have seen it happen in front of me on the A4, where a new camera was installed just before the flyover on the eastbound carriageway. A Mercedes panic braked, skidded across 3 lanes and crashed into some cars. One view is that he should not have been exceeding the speed limit. My view is that on this stretch of road the speed limit is far too low (40mph), and, in any case, a 6 lane dual carriageway is no place to be enforcing an arbritary speed limit.

There are some very old programmes around, largely looked after and updated by people like TRL.

Does not surprise me.

Walking and cycling are not mobility modes, which means that they are less modellable than motors. For instance, one cyclist may ride in the gutter in an intimidated and victimised way, whereas another may take the primary position and not be overtakeable. Pedestrians may step out into the street randomly and cause vehicular flow to stop. It's not programming this so that it occurs in a model that's the problem, but actually getting a handle on how to quantify it/what is likely to happen. It can seriously affect the accuracy of a model. It's one reason why cycling is often a mere afterthought in the design process and why we're generally not even getting cycle audits after schemes that didn't consider cycling had been designed.

I understand. I have a few ideas of how it could be quantifiable but unfortunately not enough time to try them all out. Maybe I should try to get a job doing some urban traffic modeling.:)

I understand. I have a few ideas of how it could be quantifiable but unfortunately not enough time to try them all out. Maybe I should try to get a job doing some urban traffic modeling.:)

Go for it. We definitely need more sensible people doing that sort of work. Ideally, of course, the role of modelling would be reduced in those places where the streets have more of a function than just a transport function, e.g. in a city. Back on topic, ba-damm! ;)

But if the aim of the 20 limit is to slow them down to 30, why risk criminalising people? Why not just enforce the 30 limit?/quote]

That is of course not the aim of a 20mph limit. Yes, it should be enforced to ensure that speeds are as close to 20mph as possible. But speeding hardly ever gets enforced, and until the day that happens effectively, we want to get burst speeds down. A 20mph limit does that, both for those who actually observe it and for the speeders. Also, it's not an either-or. We want both 20mph introduced as far as possible, and 30mph and 20mph enforced.

[quote]I read that the London Ambulance Service wanted them all removed because by reducing response times, forcing ambulances to avoid certain roads, etc, they cost more lives than they save. If that is true there is no justification for them at all.

I know the journalist who did that interview. The best statement I know of by the LAS is their submission to the 2003 GLA traffic calming enquiry:

http://tinyurl.com/93areh

As usual, the reality is more complex than the simple headline, and as so often vitiated by the lack of available research. I'm not aware of the LAS calling for them all to be removed, either.

I'm not aware that there have been any real advances in car suspension since the 1960s. normal cars get wrecked by speed humps. Which, as a result, means that speed bumps are directly responsible for the massive increase in big off-roaders in cities, because of their ground clearance. I don't know if that genie could be put back in the bottle by removing humps, but putting them there definitely killed the small city car in favour of the SUV.
The increase in off-roaders is undoubtedly also partially a consequence of vertical traffic calming. Riding around, though, I get the definite impression that cars can be driven faster over vertical features than fourteen years ago when I moved to London. This is anecdotal, and if there's been no change in car suspension, all the better. I suspect that vertical features have generally been engineered more to spec in recent years, though.

You still get this when obeying the speed limit. Have you ever tried going down a road with speed humps at a constant 20mph? It is impossible. You have to brake then accelerate.
Not if I'm on a bike. :) I've never driven, so I don't have first-hand experience. It would of course be better if cars travelled at a constant speed rather than speeding up and slowing down. If people do 20mph, the difference between the higher and lower speeds is quite small, though.

I have seen it happen in front of me on the A4, where a new camera was installed just before the flyover on the eastbound carriageway. A Mercedes panic braked, skidded across 3 lanes and crashed into some cars. One view is that he should not have been exceeding the speed limit. My view is that on this stretch of road the speed limit is far too low (40mph), and, in any case, a 6 lane dual carriageway is no place to be enforcing an arbritary speed limit.
Take your pick--get a higher speed limit and get the road congested within minutes in the peak hour, or choose a lower limit and keep it moving. In the case of motorways and major A roads, the reason why the limit is there is often to a large extent to increase motor traffic capacity. You'll find that this sort of speed limit will be far from arbitrary but based on careful modelling.