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@TheBrick(Tommy)
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.