Local buses tend to use the slow lane, which in North America means the rightmost lane; this is how they access the curb to pick up passengers. New York’s painted bus lanes on First and Second Avenues are to the right, with the buses slower than the cars both in perception and in actual practice.
Occasionally, transit uses the fast lane, especially if it’s BRT or a streetcar; for some of the access challenges of boarding not from the curb, see an old Human Transit thread on the subject. The issue of whether there should be sidewalk- or median-adjacent transit lanes came up in comments on Cap’n Transit’s blog. So let me explain why higher-grade transit than local buses, which means rail or BRT, should run in the median, with boarding from raised curbs either on the sides or in the center.
1. Service identity. This is probably the overarching concern, especially on the question of whether to have raised curbs or instead stop traffic in the slow lanes and have people cross to the bus from the sidewalk. ITDP’s magnum opus standards for full-fat BRT virtually take median running for granted, and only consider alternatives when the right-of-way is constrained. This is also mentioned as the highest grade of BRT in a conference paper examining BRT on city streets.
2. Fewer conflicts. Using pedestrian-friendly two-phase stoplights, it is impossible to eliminate turn conflicts, though in Delhi they found that median running (right lane in India) had fewer turn conflicts. In addition, it’s possible to eliminate conflicts with cars entering or exiting the parking lanes, as well as stopped cars left near the curbside lanes.
3. Median lanes are politically easier to physically separate, since separating them does not deprive cars of curbside access. If cars can physically violate transit lanes, they will, either accidentally or intentionally (my mother’s car’s GPS guidance routinely sends her along the tram-only lanes). As APTA mentions in its own standards for BRT,
One major advantage of a median busway is that there is typically no demand for other vehicles to stop in the center of the street for purposes such as parking or as a breakdown lane. As a result, there is a lot less reason for vehicles to want to occupy the center of the road and less resistance to creating a physical barrier separation between the busway and the adjacent general traffic lanes.
Point #3 is what killed the proposal for the 34th Street Transitway, which would have run two-way on one side of the street with one direction running contraflow. The NIMBYs on East 34th Street complained specifically about curbside access, using such language as “Delivery and service trucks… no longer have direct access to buildings and stores along stretches of 34th Street.” Most issues they raised involved curbside access or else bus noise adjacent to the street, both of which would have been solved by median lanes.
To add to what Steve Stofka is writing about grids, if I had to design a street from scratch, it would look a lot like a two-way version of a Manhattan avenue, with bus lanes in the middle. It would be 30 meters building to building, and about 20 curb to curb; this is enough space for two parking/loading lanes (2.5 meters) buffering pedestrians from moving traffic, two car travel lanes (3 meters), and two median bus lanes (3-3.5 meters), with room left for physical separation (measured in centimeters). Raised curbs for stations should add 3-4 meters, at the expense of either parking or sidewalk space once every few hundred meters; one advantage of trams, or buses with doors on both sides, is that they can use less space-consuming island stations.