Passengers per hour per direction by transit technology
Transit modes stack in order of capacity. Bus rapid transit or conventional bus achieves roughly 5,000 passengers per hour per direction (pphpd), limited by the frequency buses can depart and the number of passengers per vehicle. Light rail (street cars, trams, modern streetcars) achieves 10,000-15,000 pphpd with longer vehicles and signal systems that space them closer. Heavy rail (subway) achieves 50,000+ pphpd with fully grade-separated rights-of-way, high-frequency signaling, and long trains with many cars.
These capacities assume full utilization (packed trains) and modern signaling. Legacy systems with older signaling might achieve 70-80% of these figures. Cities facing capacity constraints either upgrade signaling systems to run trains more frequently, extend train lengths, or add parallel lines. Tokyo's packed rush hour subways approach the theoretical maximum of this technology by using both strategies simultaneously.
Capital cost and service area trade-offs
Heavy rail requires substantial capital investment: modern metro construction costs $500 million to $2 billion per mile depending on depth and geology. Light rail costs $50-200 million per mile. Bus rapid transit costs $10-50 million per mile. These differences mean that high-capacity modes are economically justified only where demand is very dense (downtown corridors, peak-hour travel), while lower-capacity modes serve lower-demand areas or areas where capital constraints are binding.
The choice of technology is rarely based on pure capacity optimization but on the density-capital trade-off: sparse cities choose bus, medium-density cities choose light rail, and very dense cities choose heavy rail. However, capacity constraints often force upgrades. London, Paris, and New York all operate heavy rail because their urban density demanded it; newer cities like Miami (light rail) or sprawling metros (Houston, Atlanta) chose cheaper modes suited to their geography. The hierarchy matters most in regions choosing new investments: undersizing today forces expensive capacity additions later.