Taxonomy and description

Bus priority measures aim to reduce journey times and improve the reliability of bus services. Measures tend to fall into four categories:

• Segregation – which gives the greatest level of priority and includes bus-ways, bus-only streets and bus lanes that take capacity away from general traffic;
• Traffic management – such as bus gates and other physical measures to assist bus movements;
• Traffic signal control; and
• Bus stop improvements.

Each of these categories of measures are likely to deliver benefits to bus users, but comprehensive improvements that involve all possible techniques are likely to produce the greatest impacts.

As these measures take road capacity away from general traffic, they can increase traffic congestion unless they are designed as part of an overall transport strategy. In some cases, there is sufficient modal shift from car to bus to reduce traffic demand and off-set the potential delays to private vehicles. If there is significant transfer from car then bus priority measures can lead to improved journey times and reliability for both bus users and car drivers.

With-flow bus lanes are the most common form of bus lane in the UK. Generally they consist of a reserved kerbside lane on the approaches to junctions which allows buses to ‘by-pass’ queues of general traffic. Sometimes offside or centre of the carriageway with flow-bus lanes are used to assist right turning movements or to avoid obstructions.

Cyclists, taxis and commuter coaches are usually permitted to use bus lanes. In some areas motor cycles and heavy goods vehicles are also permitted.

Bus lane width should be at least 3.0m. If cyclists are allowed to share the bus lane, the width should be extended to 4.3m or above to allow buses to overtake a slow-moving cyclist. This limits the implementation of bus lanes to areas where the road is sufficiently wide – typically 10.0m so that a goods vehicle or bus travelling in the opposite direction can safely overtake.

Buses should be able to enter the bus lane without obstruction from queuing traffic. If queues extend through successive junctions the start of the bus lane might need to be set forward. Bus lanes can be extended right up to junctions but this can significantly reduce capacity, hence lanes tend to be set back, but not so far that buses miss the first available ‘green’ period. For a roundabout, the set back can be shorter, perhaps three vehicles in length, without significantly affecting entry capacity.

Contra-flow bus lanes enable buses to avoid circuitous routes (e.g. one-way systems) but they restrict kerbside access. Sometimes contra-flow bus lanes are made self enforcing by means of a continuous kerbed island, but this can lead to in-lane congestion if a bus breaks down.

Traffic and Parking Management Measures

A bus gate is a generic term describing all forms of control which allow buses free movement, but restrict other traffic. They can literally be gates or rising bollards or short lengths of bus-only street. Similar exclusions can be created by traffic management measures such as no-entry and banned turn controls which allow buses to make a movement that is prohibited for general traffic.

Kerbside parking is a major cause of delays to buses making it difficult to approach and leave stops and requiring bus drivers to change lanes between stops. As it is often impractical to ban parking, some highway authorities have enabled rear servicing to commercial properties and dropped kerbs that allow residents to gain access over the footway to parking within the curtilage of their properties.

Traffic Signal Control

Methods for providing bus priority at traffic signals can be grouped into:

• Passive priority - traffic signal timings are designed to provide some benefit to buses without any means of detection. This is not effective when all arms of the junction are bus routes.

• Active priority - traffic signal timings are modified to provide priority to individual approaching buses. The detector is usually placed 60-100m ahead of the stop line and either extends the green phase or brings forward a green phase, depending on point in the sequence.

• Congestion management - traffic signals manage congestion by ‘holding’ queues where they have limited impact on buses (e.g. where a bus lane might be provided). This includes providing pre-signals and bus advance areas that allow buses to get ahead of general traffic at a junction.

Bus Stop Improvements

Inconsiderate parking at bus stops can delay buses, prevent vehicles getting close to the kerb to assist passengers boarding and alighting, and impact on passing traffic. Bus stop clearway markings can help to deter obstructions, but they take up a length of carriageway that is also needed for residents’ parking or commercial deliveries.

Bus boarders (or built out bus stops) are local extensions of the footway into the carriageway that perform a similar function to clearways, but typically take up an area 2.0m wide and 9.0m long. Boarders allow buses to stop outside the line of parked vehicles, passengers to board and alight directly onto a pavement, and buses to re-enter the stream of traffic.

Bus stop locations must not unduly block general traffic and for this reason may need to be provided with half-width (1.5m) bus bays which give more space to general traffic whilst still allowing easy access and egress for buses to and from the main carriageway.