Cities are generally built on flat terrain. There are some famous exceptions [0] but for obvious reasons the slopes of streets within city limits tend to be tractable. With the e-bike revolution one can also address the edge cases (historic cities, elderly populations etc)

So there is a massive opportunity to de-congest cities by shifting to (e-)bike transport. The key obstacles I see are:

* weather conditions. unfortunately bikes are not particularly practical in harsh winter conditions. So the goldilocks bike zone is not available to all. inventing potential means to extend it would be doing god's work.

* cultural. while oversized monster trucks are a particularly american obsession, the fact is that the common car has become a symbol of affluence and convenience across the planet. bikes were always a poor man's transport. politicians riding bikes still make people go "wow - how humble". the challenge is to change this perception

* vested interests. a good fraction of the global economy revolves around cars. an optimal reconfiguration of cities would not eliminate cars, but it would still significantly reduce the numbers in circulation. there will be resistance for as long as people can neglect the externality of wasting enormous resources for no good reason

[0] https://en.wikipedia.org/wiki/List_of_steepest_roads_and_str...