Many advocates claim that LRT has higher passenger capacity than lower-cost alternatives like BRT. A closer review of available research shows this to be an often repeated misconception that is unsubstantiated by recent real-world experiences. Using transit best-practices (like interlining and passing lanes at BRT stations) can substantially increase BRT infrastructure capacity as measured by people per peak hour per direction (PPHPD). In addition, using new technology like automated vehicles and double-articulated hybrid buses (eg Vossloh Kiepe and Hess) with 250 passenger capacity can further expand BRT capacity and efficiency.
When the TransMilenio system in Bogotá, Colombia, opened in 1998, it changed the paradigm for limited BRT capacities by providing a lane for buses to pass each other at each station and multiple sub-stops at each station; and by introducing express services within the BRT infrastructure. These innovations increased the maximum achieved capacity of a BRT system to 35,000 PPHPD. Light rail, by comparison, has a maximum theoretical capacity of about 20,000 PPHPD, but these levels have rarely if ever been achieved under real-world conditions, and they require very long multicar vehicles on fully grade-separated rights-of-way (either elevated, as in Manila, the Philippines, or underground). On normal city streets, the highest-capacity LRT systems are in Europe, and they typically carry a maximum of about 9,000 PPHPD. There are conditions that favor LRT over BRT, but they are fairly narrow. Meeting these conditions would require a corridor with only one available lane in each direction, more than 16,000 but fewer than 20,000 PPHPD, and a long block length, so the train does not block intersections. These specific conditions are rare, but where they exist, light rail would have an operational advantage. Otherwise, any perceived advantages of LRT over BRT are primarily aesthetic and political rather than technical. — ITDP study, More Development For Your Transit Dollar
In the US, current transit capacities are significantly lower than those of the BRT and LRT systems mentioned above. This is because domestic capacity is measured as a function of the number of vehicles currently serving the corridor (at peak hour, in peak direction), and the physical capacity of those vehicles. Yet no corridor in the US has sufficient demand to justify vehicular frequencies high enough to saturate the corridor. For example, the current capacity of Los Angeles’ Orange Line BRT is 1,965 PPHPD based on the existing fleet. However, the system’s theoretical capacity is much higher: were demand to grow and more vehicles put into service, capacity would increase. The LRT corridors in Los Angeles—the Gold Line and the Blue Line—have similar capacities based on the existing fleet: 2,090 PPHPD. This capacity, too, could grow with an increase in demand. Note, however, that in order to provide capacities that more or less meet current demand, Los Angeles provides less frequent services on its LRT lines due to the size of the LRT vehicles.
US cities generally search for the sweet spot in the demand-to-capacity ratio and try not to provide service frequencies that are so high that their vehicles run empty. Thus, since LRT vehicles are larger, in order to justify providing LRT capacities that are similar to a BRT, LRT tends to operate at lower frequencies. As mentioned above, due to the perceived capacity constraint of BRT there are currently no cases in the US where LRT should be favored over BRT. — ITDP study, More Development For Your Transit Dollar