Designing Bus Route and Schedules
This article is the third in a series of articles about how bus routes and schedules get designed, from the initial determination of the route to ongoing maintenance of the schedule. Practices mentioned in the articles are ones that I engage in on a daily basis that I have found to be useful in my twenty-eight years of studying and following transit systems. TCRP Report 100 , put out by the federal government, is essential reading on this subject.
Part III: Determination of Times for the Bus
This is the third section in how bus routes and schedules get designed. Recall in Part I we determined the placement of the route and in part II we looked at how bus stop locations are determined. In this part we will look at how we determine the time points of the route and how long the bus should take to drive between the time points.
The building blocks of a bus schedule are the time points and the running time in between them. Each route will at the very least have two timing points: the start and end of the route. If bus drivers will be relieved at a particular location, then that location must be a time point as well. Additional time points should be added so that there is a time point approximately every 8 - 10 minutes of driving time, with more time points at the beginning versus the end of the route. Good locations for time points are major transfer points and significant trip generators - bus stops that are responsible for more than 10% of the total boardings for the entire route are good candidates for time points. It is very important that an optimum number of time points be selected for the route. Too many time points works to slow the overall bus speed down as drivers may drive excessively slowly in order to not arrive or depart from a time point early. Too few time points can encourage bus bunching as drivers may drive at different speeds from one another.
Once time points are tentatively selected, the route needs to be driven in order to determine the running times required between each time point. A good rule of thumb I use to estimate initial running time is to drive the route at a maximum speed of five mph below the speed limit. After I obtain the time required to drive the route in this way I multiply the total by 1.3 to account for the time spent at bus stops picking and dropping off passengers. First, the entire route needs to be driven and the overall time, including the 1.3 multiplication factor, required to drive the route calculated. Next, the time required to drive between each time point should be calculated. Compare the summation of the time required to drive between each time point with the overall time required to drive the entire route; if the summation of the individual time point times is greater than the overall time then subtract running time evenly from the time required to drive between each time point, starting with subtracting time from the last time point before the route terminus to the terminus. If the summation of the individual time point times is less than the overall time then add running time evenly from the time required to drive between each time point, starting with adding time from the start of the route to the first time point after the start and so on. Distributing slightly more running time to the first half or the route works to keep the route on time and improves the on-time percentage for people who are boarding early in the route, and most boardings along a route occur in the first half. Decreasing running time slightly later in the route reduces the chance that a bus will have to wait at a time point with few boardings, and improves the chance that a driver will arrive at the terminus in time to take a break. Express route running time determination will not use the 1.3 multiplication factor during the express section, and limited stop routes should instead use the actual running time plus 30 seconds multiplied by the number of stops as opposed to the 1.3 multiplication factor.
Normally, the route should be driven at several different times during the day in order to take into account the fact that running time varies by the time of day. At a minimum, the route should be driven during the A.M. peak period around 8 A.M., during the mid-day around 12 PM, during the P.M. peak period around 4:30 P.M., and at night around 8 P.M. On weekends the route should be driven twice: around 8 A.M. for the early A.M. / night time and around 2 PM for the afternoon time.
After the running time is successfully calculate the overall cycle time can be determined. The cycle time is generally defined as the overall round trip running time plus any required layover at the end of the route. Layover, where the bus waits at the terminus location before starting the next trip, serves two major purposes: it helps to maintain on-time performance and it serves to give the driver a break. Some transit systems schedule regular breaks for drivers at garages or other rest points. At these companies, a layover percentage equal to 10% or less of the total one-way trip time is adequate to make sure the return trip leaves on time. At other systems, the layover is the driver break time. At these systems I generally desire to have a layover percentage at the end of the line between 15 and 20% of the total one-way trip time. If the one-way trip time is relatively short, less than thirty minutes, then almost all the layover time may optimally be scheduled at one end of the route as long as at least a couple of minutes are scheduled at the other end to ensure on-time performance.
When the cycle time is determined you are now able to determine how many resources the route will take to operate. Most transit systems decide the frequency of the route; once the frequency is determined the number of buses required to operate the route follow from simply dividing the cycle time by the frequency. Other systems, notably Toronto, determine how many buses will operate along the route and determine the frequency by dividing the cycle time by the number of buses.
Since the majority of transit system only operate routes on frequencies that are divisible by 60 (i.e. buses every 10, 15, 20, 30 or 60 minutes), it is desirable to operate routes that have cycle times that are in multiples of 30 - 60. Cycle times that do not fit this pattern will result in layover percentages that are either too high or too low, which will result in unproductive layover time or deadheading to different locations if interlining is not an option.
Now that we know how long the bus will take to travel along our route and an idea as to how many resources will be required to operate the route, we are now able to write the schedule. In Part IV of this series I will examine how transit schedulers actually write the schedules that you look at when you are waiting at the bus stop.
