How to design a city?

Surely you would define congestion as lots of closely packed cars driving slowly. Ergo, slowing cars down, perhaps by means of a 20mph limit will make congestion worse. I doubt that traffic flow would improve. You'd still have the same stop-start burst speed thing as now, just slower longer-lasting. 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)

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.

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.

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. 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!). Ergo public transport will always be a less desirable option to those who drive a car, no matter how much you spend on it. 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.

This is why I have - until recently stayed out of this discussion.

Lots of long posts with untruths and uneducated assumptions.