Other countries prove that 2-wheel transport can make up the mainstream. I've ridden on main roads in bicycle "schools of fish", and that didn't require closing the road either, it was daily life!!

The number of wheels isn't the main safety issue, it's speed and the size of vehicles on the road.

Rather than reserving roads for cyclists and pedestrians, I would reduce speed limits and discourage the use of heavy vehicles.

"the number of wheels isn't the main safety issue, it's speed and the size of vehicles"
MoreBikesPlease2,I agree with you on speed and size but having the car handling abilities of a 3 or 4 wheeler is crucial. In a moment of truth when someone cuts you off at 25mph in a motorized bike, I would rather be in what is essentially a bicycle car.
My speed limit would be 25mph roads and main streets and 35 on freeways or freeway sections totally sealed off to any other traffic. Needless to say I'm picturing a post car world still using whatever pathetically designed infrastructure that's left over.
In the world of my dreams everything would be redesigned drastically and private vehicles would neither be cars-Rube Goldberg design-or mere feeble bikes. They would be ultralite,amorphous metal
rollover bodies, like dune buggies. Safe microcars.