It's much harder to try to plan a bus station from the top down. I have always planned them from the bottom up, based on the planned service:
1. Start with the number of bus routes to be served.
2. Consider whether bus arrivals and departures will be "pulsed" (that is when many routes arrive or depart at the same time, which allows easy transfers when buses are relatively infrequent -- half-hourly or less often).
3. If buses are not "pulsed", then depending on the frequency and reliability of service, multiple routes can share berths.
4. Decide whether bus routes are frequent enough to be assigned berths, versus taking the first open berth. The latter approach requires some means of communicating to passengers where to go to catch their buses. People with sensory disabilities (poor vision and/or hearing) benefit from being able to catch the same bus at the same place all the time.
5. Determine the bus specifications, particularly the length. A typical 40' (12m) transit bus requires about 55 to 56' (17m) outside turning radius for comfortable turns without backing up, though tighter turns (50' or 15m) may be feasible if the site is really constrained. Lay out the bus circulation based on these specifications.
6. Consider the need for some buses to pull in or pull out ahead of or behind other buses.
7. Allow extra space for buses to queue while entering and exiting the site, depending on traffic on the adjacent roadways.
8. For buses equipped with wheelchair ramps, a minimum clear platform width for passengers is 8' (2.5m), although I have found 10' to 11' (3 - 3.5m) can better accommodate wheelchair movement.
9. Add to the minimum platform width as necessary to accommodate passenger amenities such as seating and trash receptacles, shelter structures, signs, ticket machines, other vendors, and so on. Double-sided platforms may be able to share space, as well.
10. Green space can fill in the otherwise unused area. If you need to accommodate storm water infiltration on site, allow about 10% of the paved area for drainage swales or basins (that is based on Sacramento, California, which has just a little more annual rainfall than Kabul).
As you can tell, the space needs will vary depending on the desired level of bus service and the geometry of the site.
The other way you can proceed is to find an available, well-located piece of property and squeeze a bus station on it as best as possible. But the level of bus service may be constrained by the available space.
I hope this helps. Good luck!
- David Solomon, AIA, CASp
State of California Department of General Services