Clearly the best way to increase ATN capacity is to reduce the minimum headway. Many experts believe headways as low as 0.5 seconds should be safely achievable – human drivers with relatively slow reflexes achieve headways under one second at speeds over 60 mph every day on our freeways. A headway of one second with a four-passenger vehicle results in a maximum theoretical line capacity of 14,400 passengers per hour per direction (pphpd). So why has it not yet been achieved?

A large part of the problem is the common, but antiquated, safety requirement that the preceding vehicle must be assumed capable of stopping instantaneously – the so-called “brick wall stop requirement” which assumes the preceding vehicle can instantaneously turn into a brick wall. After many years, ATRA succeeded in persuading the ANSI/ASCE/T&DI Automated People Mover Standards to allow ATN systems to meet alternative requirements. These were codified in the 2021 edition. No system is yet known to have received safety certification under the new requirements.

While control and communication systems can fairly easily achieve safe operations with headways under one second, proving that the mechanical elements of a system can do so is more difficult. It must be shown that a mechanical failure cannot lead to higher stopping rates than the following vehicle can achieve. Vehicles relying on friction between rubber tires and pavements have difficulty meeting this requirement because the friction is subject to degradation due to water, snow, ice, or even leaves, on the track. This is why Ed Anderson developed Taxi 2000 using linear induction motors for both acceleration and deceleration. 

Two other ways of achieving higher capacity are:

1. Doubling guideways. This is like adding lanes to a road and has been proposed by Ultra in India. One drawback is the extra capital cost. Another is the need for more complicated guideway layouts.
2. Platooning vehicles together. Modutram in Mexico has this capability. They also use eight-passenger vehicles – their system is more like group rapid transit than personal rapid transit. Their theoretical maximum capacity at six-second headways is 4,800 pphpd with one vehicle and 19,200 (like light rail) with a train of four vehicles. Modutram can automatically couple and decouple stationary vehicles. Independent vehicles sharing the same route and initial destinations can arrive at a station, form a train and depart together. At one of their initial stops, they can decouple and proceed independently to separate destinations on different routes.

While the platooning process is superior to legacy automated people mover or train systems, on a large network with many stations it is inferior to a PRT system with facilitated ridesharing.

The primary advantage of these two options is high capacity without requiring very short headways.