The source of the disconnect between San Francisco's transit-first heart and its car-centric hand is an arcane engineering measure called "level of service," or LOS. In brief, LOS suggests that whenever the city wants to change some element of a street — say by adding a bike lane or even just painting a crosswalk — it should calculate the effect that change will have on car traffic. If the change produces too much congestion, then a great deal of time, money, and additional analysis must go toward the project's consideration.