I haven't ridden in San Francisco, but my experience in urban areas is that my speed may vary from 8-17+mph. The difference depends on traffic, time of day (traffic), route and weather more than clothing. The 2 mile rides were always the slowest (across town in congested rush hour traffic), and often took almost as long as the 7 mile rides on weekends.

Personally, I like long pants more to protect against sunburn (and cold in the winter) than falls; other riders with longer commutes have different preferences.