More realistic logic for pedestrians starting to cross

Copying partly from #485:

Turn requests are only made when a pedestrian/vehicle is at the end of the queue, ready to start the turn. And vehicles can instantly start/stop. So that means that at a big road, a pedestrian might reach a crosswalk a moment before a vehicle gets to the end of the queue. Even once we have the notion of a crosswalk where pedestrians yield to cars, the pedestrian won't see any conflicting turns and will just immediately proceed. We should fix that (in a separate issue; just starting the write-up here). I think it's only slightly tricky -- the lead vehicle in a queue will call ahead to the intersection and make the request. If the turn has higher priority but the vehicle hasn't arrived yet, then we have to calculate the vehicle's ETA to the intersection and figure out if there would be a conflict. Actually, the lead vehicle can give the ETA when they first make the advanced request. Because they're the lead vehicle and they know their own speed, it should be easy to estimate... I guess the only case is if there's another vehicle doing the same turn that they're blocked behind. Should work out; the ETA is a best case.

And @BruceBrown has some ideas about batching together a few pedestrians to cross together.

Also we identified that `rand_ped_speed` shouldn't be a uniform distribution between 2 and 3mph; most people are closer to 3mph.