What Is the Capacity of Different Modes of Transit?
Many times when we read stories about new public transit project one of the things we read about is how a certain mode will not provide enough capacity for the expected ridership, while another mode may provide too much capacity for the expected ridership. What do we mean when we discuss capacity of different modes of transit?
The capacity of a transit mode refers to how many passengers per hour a mode can be expected to carry. Since when we discuss capacity we are usually discussing it in terms of a rapid transit project, the capacity should be defined as the maximum number of passengers per hour a given mode could carry at its maximum average operating speed. We can visualize this in terms of an expressway: a gridlocked expressway will have more cars per unit area than one at free flow, but this fact does not mean that the gridlock represents the capacity of the freeway, because the freeway is not designed to operate at a state of gridlock
Overall, the capacity of a given transit mode expressed in passengers per hour can be represented as the result of multiplying the number of vehicle sets (trains) that could pass by a particular stop in one hour (the frequency) by the number of vehicles per train and the number of passengers that could be carried by each vehicle. I shall discuss each factor in turn.
Maximum Frequency of Transit Vehicle Sets (Trains)
The maximum frequency of trains operating in a rapid-transit like setting depends on if they are operating at grade or they are grade-separated. Since in order to maximize average speed vehicles operating at grade need to have traffic signal priority, the maximum frequency of trains operating at grade depends on the signal priority. For signal priority to work effectively, trains can pass by the signal no more than once every four minutes so that the other traffic has a chance to proceed as well. While of course trains operating at grade can operate more than every four minutes, doing so will result in some of the trains being forced to stop at red lights - causing delay. Readers who are familiar with streetcar routes in Toronto operating along streets with traffic signal priority and operating more frequently than every four minutes - such as Spadina - will no doubt recall times when their vehicle has been forced to stop for red lights.
In a grade-separated setting, the maximum frequency of transit vehicles is determined mainly be signalization, turn-around time at the route termini, and dwell time at the busiest stations. In general, the above factors mean that fully maxed out a grade-separated transit vehicle could operate every two minutes, although fully-automated trains, such as Vancouver's SkyTrain, can operate as frequently as every ninety seconds. Attempting to operate more frequently than this, even if allowed to be the block signals, will likely result in bottlenecks at very busy and terminal stations.
Number of Vehicles Per Train
In an at-grade system, the maximum number of vehicles per train is usually three due to the requirement that the train not block intersections when stopped at a red light or at a station. In a grade-separated setting, the maximum number of vehicles per train is determined by how long the station platforms are. Most subway systems allow for a maximum of six sixty-feet cars per train, although some - especially BART, which can have up to ten-car trains - have longer consists, while others - especially Vancouver's new Canada Line, which has only four car trains - have shorter consists.
Number of Passengers Per Vehicle
The other factor that affects how many passengers can be carried by transit is the number of passengers that can fit on each vehicle - a number that is represented in transit by the load factor . While in buses load factor is usually limited to a maximum of 1.5 - meaning that all of the seats are filled and there are standees equal in number to half of the seats - rail vehicles, which are often designed to have additional standee spaces, can have a higher load factor of 2.0 or even higher. For the sake of this article I will assume that a high-floor subway car could carry 100 passengers per vehicle while a low-floor articulated bus or light rail car could carry 90 passengers per vehicle.
Capacity of Different Modes of Transit
Now we are ready to calculate the capacity of the different modes of rapid transit.
Bus Rapid Transit (at grade)
90 passengers per vehicle * 15 vehicles per hour = 1,350 passengers per hour per direction. This number suggests a maximum daily ridership of around 20,000, which is what the Los Angeles Metro Orange Line is averaging.
Bus Rapid Transit (grade-separated)
90 passengers per vehicle * 30 vehicles per hour = 2,700 passengers per hour per direction. Note that by lengthening the platforms at bus rapid transit stations to provide more than one space where a bus can stop you can add more vehicles and thus more capacity.
Light Rail Transit (at grade)
90 passengers per vehicle * 3 vehicles per train * 15 vehicle sets per hour = 4,050 passengers per hour. This number suggests a maximum daily ridership of around 60,000.
Light Rail Transit (grade-separated)
90 passengers per vehicle * 3 vehicles per train * 30 vehicle sets per hour = 8,100 passengers per hour.
100 passengers per vehicle * 10 vehicles per train * 30 vehicle sets per hour = 30,000 passengers per hour. This number suggests a maximum daily ridership of around 450,000. The Bloor line in Toronto has a daily ridership of almost 500,000, while the Yonge line, which is really two lines - Yonge and University-Spading - has a ridership of over 700,000.
The above numbers assume lines with only one peak load point; i.e., with no turnover of passengers. In addition, the numbers are meant as a general guide only so you can see the magnitude of the difference in capacities amongst the different modes. With the exception of the biggest cities in the United States and Canada, no city will have enough demand to justify the cost of construction of grade-separated rapid transit . In the case of the biggest cities, care must be taken not to construct a line that does not have enough capacity to meet long-term demand. Los Angeles is perhaps the most guilty of this problem, with both the Orange Line and Blue Line at capacity.