- The average speed of the line (commercial speed)
- The frequency of the line
|Graph illustrating the relation between frequency and ridership|
Unfortunately, that is not quite accurate, at least, not with regular local bus service (streetcars have the same issue most of the time).
|Average speed of a transit line versus the average distance between stops|
For miles per hour, divide by 1,6.
OK, a bus can still get a decent speed despite a great number of bus stops if most of these are unused, as it doesn't need to stop at each bus stop, only at those where people get in or get out. That means that the less people per kilometer board or get off the bus, the faster the bus will go.
|Average speed of urban buses versus the number of boarding per km, supposing bus stop spacing of 200 meters|
|Average speed of suburban buses versus the number of boarding per km, supposing bus stop spacing of 200 meters|
Note that the reality is even worse than that, as bus drivers are on a schedule, and the only way to maintain that schedule is to build in excess time so that buses can catch up to the schedule if there are more passengers than expected. What this means is that if there are less passengers than expected, bus drivers will slow down not to arrive too quickly to the next stop, since being late is one thing, but being too quick is more than just a little annoying to transit users who see the bus passing before them as they're walking to the stop.
What this underlines is the importance of rapid transit investment with grade separation so that transit can be at the same time high-capacity and high-speed, which is necessary to compete effectively with cars. To satisfy oneself with simple buses running on shared streets is to be satisfied with transit remaining a marginal mode of transport which will take 2 to 3 times more time to get anywhere, in the best case scenario.
|Average speed of an urban bus line depending on boardings and stop spacing|
Greater stop spacing does mean that people will have to walk greater distances, but it's a trade that's worth it if the goal is to make transit a competitive way of getting around and to increase ridership rather than to perpetuate the conception of transit as a form of welfare program made exclusively for those too young, too poor, too old or too sick to drive.
As an added bonus, if buses are faster, as most of the cost of running them is in the form of wages, it means that their costs tend to be proportional to their time running rather than distance traveled. So if you speed up buses, that means that you can increase the frequency of lines for a negligible increase in costs as the buses finish their routes faster and thus can be sent back to do another route quicker. If you can afford 4 separate buses on a route and this route takes 40 minutes to run, then that means each bus can only do 1,5 time the route each hour, for a frequency of one bus every 10 minutes, 6 buses in an hour. But if you speed up the buses by 30%, then the route takes only 30 minutes to do, the same 4 buses can do the route twice every hour, for an interval of 7,5 minutes between buses, with 8 buses an hour from the point of view of riders. This increases frequency and capacity of the line, all for a minimal additional cost (mostly gas).