Originally Posted by Helmet Head
Just to be clear, by "multimodal" you mean trimodal (cars, bikes, peds) as opposed to bimodal (vehicular, peds), right?
It's not an engineering problem. It's a basic math problem. Assuming you want to provide continuous segregated routes, as much as possible, for each mode, it's the intersections that are the problem. The intersections are a problem for just a single mode problem. The engineering problem is that we're trying to do this on essentially a single plane. That means you have to stop certain flows across an intersection to allow another through (stop signs, traffic signals). The alternative is much more expensive; grade separation.
When you add a second mode to the problem, that complicates the problem. Others understand this, and so seek to eliminate cars, for example.
But adding a third mode, that's really impractical. You either have impractically expensive multi-grade solutions at intersections, or you have to allow mode-specific flows at intersections, adding delay for everyone. Again, this is already a problem in just a bimodal system (consider how many peds are killed per year).
Adding a third mode to bimodal transportation system complicates everything by probably an order of magnitude. That makes it impractical.
Actually you were right when you said it is a math problem... and a very simple one. Slow down cars, and cars and bikes will mix very well. It is only when motorists are trying to achieve near freeway speeds that the broad mix of vehicles tends to fail. Residential streets and 25MPH downtown areas tend to work very well.
If motorized transportation was limited in speed in certain areas, those areas could easily support all sorts of modes of transit, from ped to car.
If these areas were also paralleled with a high speed system (freeways) then motorists could use the capability of their vehicles to cover large distances easily.
It simply comes down to doing things at a human scale, not a car scale.
Perhaps it is only a matter of time before wireless speed control governors are installed in motor vehicles to limit their speeds in urban surface street environments. (only one of many possible solutions)