How to design a city?

@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.