Starter project: Improve traffic signal heuristics

I can't find data about how Seattle traffic signals are actually timed right now. So for the moment, I'm automatically generating a policy for every intersection. There are a bunch of different policies for different types of intersections (for a 4-way, should there be 2 phases with no protected left turn or 4 phases with protected left turns?). Every phase is hardcoded to 30 seconds right now. A nice project contained to one layer of the code (just the map model) would be to improve these heuristics. For example, roads have a rank (primary > secondary > residential) from OpenStreetMap, so probably bigger roads should have longer durations.

Ex policy: https://github.com/dabreegster/abstreet/blob/573edbd13e5aa4bedba0bd819d3d7b32dba11b59/map_model/src/traffic_signals.rs#L247