Low-demand times and locations, however, quickly bring conventional public transport to the limits of efficient operation, resulting in insufficient or no service being offered and most potential customers turning to automobile use in response. With this in mind, different forms of flexible (in terms of time and location) service are being developed and tested to supplement scheduled service. Smaller vehicles such as minibuses and passenger cars are being used to offer demand-responsive forms of service in which the customer is required to pre-book, eliminating costly empty runs.
The systems are defined by degree of flexibility, with fixed-route, dial-a-ride, deviated route, and flex zone services offered.
In deviated route services, it is possible to differentiate between fixed service with defined routes and limited scope for deviation, point deviation with a few designated (e.g. start and end) stops as well as requested stops for passengers within a predefined area along a fixed alignment or route, and purely on-demand services. As in the case of flex zones, this may include door to door service.
There are a multitude of names for these services, including demand-response buses, shuttles and vans, dial-a-ride, metro access, link-up services, share taxis, event shuttles, etc. Passengers usually make reservations by telephone, but text message and e-mail are also used.
Scheduling and dispatching of dial-a-ride services can be handled by central dispatchers in transport company operation centres, mobility centres for local and regional bookings, or taxi dispatch offices. The operators use fleet telematics technology, rapid response to passenger requests, and planning of routes based on current demand to optimise these flexible services to the fullest extent possible. Innovative technology allows the shortening of advance reservation times, minimisation of waiting times and circuitous routes, maximisation of vehicle occupancy, significant reduction of personnel costs for recording and processing passenger requests and vehicle dispatch, and improvement of reliability and punctuality of service.
From the dispatch centre, ride requests and last-minute changes and additions are communicated to the vehicles, usually by means of mobile phone or radio or data transmission.
There are also more elaborate technical solutions incorporating on-board computers for high traffic volumes or more flexible systems. When multiple vehicles serve a zone with no fixed route or timetable, computerised scheduling optimisation is necessary to keep travel and waiting times short while still utilising capacity as efficiently as possible.