Transit assignment problems are, in
many ways, more difficult than passenger vehicle assignment. The public transport
network is different from that of private cars. It contains sections of the bus,
rail, or ferry services, as links, running between two stops, stations, or
terminals. Link capacity in public transport is related to the capacity of each
bus, train, or ferry and its corresponding frequency. The total travel time includes
an in-vehicle time as well as components for waiting at stops and walking to
and from them. Many of the public-transport sections will use road links, while
there will be other public-transport sections which use completely different
links, e.g. busways, segregated rail track, etc. These links generally produces
a more complex network. In public-transport route choice we are dealing with passengers
and not vehicles.
Passengers can drive part of the way
to board a public-transport service, walk to a stop, and interchange between
two services. Therefore, providing walk and transfer links between different
services and different transport modes (bus, rail), and between public-and private-transport
facilities (e.g. ‘Park & Ride’) is needed.
The monetary cost in private car
networks is usually associated to fuel consumption, which is proportional to
travel distance. In public transport, payment systems based on smart cards allow
more complex fare structures such as fares variable with distance, zonal fares,
time limit fares, and other season tickets for a fixed service or covering one
or more zones and modes. This wide range of fares places difficult requirements
on route choice and assignment models, as costs depend on the location of the
origin and destination, and on the route chosen.
The ‘common lines’ problem is
probably one of the most difficult and typical problems of public transport
assignment. In the common lines problem, passengers minimize their total travel
time by deciding to board an arriving vehicle or to wait for the vehicle of another
line. The set of arriving vehicles that a passenger would board becomes the
“attractive set of lines”. The choice is therefore more complex and calls for a
more detailed treatment.