Public transport services

Taxonomy and description

Description

Public transport services refer to the entire range of transport services that are available to the public including demand responsive transport, buses, trams, light rail systems, metro (underground) and long distance rail services.  

Service levels can be defined according to a number of dimensions, the key ones being the frequency of public transport services (i.e. ‘regular interval and clockface timetables’, ‘combined service frequencies’, and reliability), the hours they operate (period of operation), where they operate and the origins and destinations they serve (both related to network coverage). Network coverage is an important issue (the density and extent of the network). If services are concentrated along main corridors then the network coverage is likely to be poor and result in lower levels of accessibility for passengers than when services have a greater spread.

Frequently consideration of means of encouraging modal shift from private vehicles involves a focus on adjusting fares to compete with cost of driving, as discussed under public transport fares. However the quality of the public transport trip can also play a crucial role in determining the entire decision process on whether to use public transport for a particular journey).  Moreover aspects of quality, especially matters of information provision and security, are crucial in enabling passengers to feel confident and safe while using public transport (SEU 2003).

In relation to transport, Metz has stated that the “quality of a journey is a function of comfort, reliability, safety, and security” (2005, p. 357). Therefore service levels are also seen as a factor in determining the quality of public transport services as they influence reliability.  Karu et al (2007) consider quality aspects in their public transport quality evaluation to be public transport reliability, security, frequency, times of operation, schedule and routes, public transport cost as well as the preference for car usage or walking.

Terminology

A change in service frequency will impact upon passenger demand in a number of ways and will have an impact on the quality of public transport provided. However in terms of terminology there are three which perhaps need to be clarified and result from a change in service frequency and impact greatly on the quality element of reliability.

Schedule Delay Time - This is the difference between when a passenger would most like to travel and the scheduled time of travel. This could involve the time spent waiting at home or at work before walking to the bus stop. The estimation of schedule delay time depends upon when passengers would ideally like to travel, the timetable and passengers’ preferences for arriving early or late. As service headway (time between two particular buses normally calculated as 60mins/number of buses per hour) increases it is likely that this element of generalised cost (the monetary and time cost of a journey) will increase in importance relative to scheduled wait time.

Schedule Wait Time - This is the time spent waiting at the stop and is usually taken to be a function of the service headway and service reliability. Where services are frequent and at regular intervals, passengers are assumed to arrive at the bus stop at random and are therefore assumed to wait on average a time equal to half the service headway. Where services are infrequent, it is assumed that passengers time their arrival at the bus stop to coincide with the arrival of the bus.

Excess Wait Time - This is additional time spent at a bus stop/station when the passenger has been unable to board the first bus due to overcrowding. This component is largely beyond the control of the passenger and is a function of the demand compared to capacity of the service.
Without knowing more about the desired departure times of passengers it is difficult to accurately estimate schedule delay time and excess wait time and for this reason it is common to find scheduled wait time as the sole representative of service frequency in the specification of generalised cost, e.g. half of the headway = (30 mins/number of buses per hour). It should be noted that wait time will also be affected by service unreliability and cancelled services. These are considered separately under bus service management.

Technology

It is technically relatively simple for bus operators to implement changes to their current service levels on a daily, weekly or monthly basis although timetables will need to be altered accordingly. A simple light rail network may also be relatively simple technically to amend in terms of service levels. Heavy rail systems with complex networks are far more difficult to amend. Heavy rail operates from highly specialised infrastructure, where safety procedures are strictly adhered to. As such there is a high level of careful co-ordination between all services that mean changes to one service have considerable knock on effects to the network as a whole. For these reasons any changes in service levels have to be co-ordinated at a network level and if there are several operators then they must involve discussions with all affected service operators. As such changes to service levels can take several months to be approved and implemented.