Definition of a Mini-Roundabout
A mini-roundabout is a type or form of junction control at which vehicles circulate around a white, reflectors, central circular road marking (central island) of between one and four metres in diameter, as shown. Vehicles entering the junction must give way to vehicles approaching from the right, circulating the central island.2 The central road marking is either flush or slightly raised (no more than 125mm), in order that it can be driven over by larger vehicles that are physically incapable of manoeuvring around it. The dome is also raised to discourage vehicles from driving over the central island . Three white arrows are painted on the carriageway, within the gyratory area, around the central road marking, showing the direction of circulation.
Use of Mini-Roundabouts
Mini-roundabouts were initially developed as a method of improving safety at existing junctions, but are now increasingly included as part of new development proposals. Mini roundabouts may be introduced at junctions that experience problems with safety or side road delay. They can be used at junctions to break up long, straight sections of road or to achieve a sharp deviation of the main route without the need for low standard radii. Mini-roundabouts are often considered as an alternative to another junction type due to constrained highway space or because they are perceived to be less costly. Early examples were used as an alternative to traffic signals at very constrained sites where an alternative method of control was needed. The four main reasons why practitioners consider mini-roundabouts as a potential option are: • to improve the operation of an existing junction; • as an accident remedial measure; • as part of a traffic calming scheme; or • to provide access to new development.
Improving the Operation of an Existing Junction
Mini-roundabouts are used to replace priority junctions, traffic signal junctions and conventional roundabouts to improve junction operation. They are usually installed at T-junctions and crossroad junctions (3 or 4-armed junctions). Mini-roundabouts should not be used at junctions with five or more arms
A mini-roundabout can improve the operation of a junction by • Reducing the dominance of one traffic flow As the mini-roundabout works on the principle of ‘priority to circulating traffic from the right’, minor traffic flow can be given priority over a major traffic flow that would otherwise dominate the junction. • Giving priority to right turners Again the ‘priority’ principle of operation has been exploited for right-turning traffic, giving it priority over ahead movements from the opposing direction. • Facilitating access and reducing delay at side roads The ‘priority to the right’ rule effectively halves the traffic to which side road flow has to yield priority, making it easier for side road traffic to turn. • Improving capacity at overloaded junctions For a given road space, the mini-roundabout has a higher capacity than most alternatives and is very flexible in coping with variations in both volumes and proportions of traffic flow during the day.
As an Accident Remedial
Measure Mini-roundabouts are most commonly introduced as an accident remedial measure: • to reduce the number of accidents at a junction. For 3-arm sites, the mean accident rate for mini-roundabouts is similar to that of priority T-junctions and about 30% less than for signalled junctions. • to reduce the severity of accidents at a junction. The severity of accidents (percentage of fatal
and serious accidents to all injury accidents) at 3-arm mini-roundabout sites is lower than at 3- arm signalled junctions and considerably lower than at 30 mph T-junctions.
The scope for accident reduction will clearly be dependent on specific junction characteristics, such as traffic flow and geometry, as well as accident types. When considering a mini-roundabout as an option, designers should refer to current guidance on accident numbers such as the MOLASSES database, and locally held records on accident levels.
As a Traffic Calming Measure
Mini-roundabouts are also used for traffic calming: • As part of a traffic calming scheme. Mini-roundabouts are often considered as part of area-wide traffic calming schemes in which they are sometimes installed at the extremities of the scheme or at all or various junctions within it. • Reducing traffic speeds and increasing driver awareness. The use of a mini-roundabout in isolation as a speed reducing measure is more contentious and has met with mixed success. They have also been used to indicate to drivers that they are entering a more residential area. A well designed mini-roundabout can reduce speeds and a poorly designed one may not.
Designers may use numerical criteria to determine whether a mini-roundabout is suitable for access to a new development, with some suggesting side road traffic flows should be not less than 500 vehicles per day (AADT). Some Local Authorities use different criteria. For example, Lancashire, Cheshire and Bedfordshire County Councils prefer to use a ratio, suggesting side road flow should be a minimum of 10-15% of the major road flow. A lower flow limit is prescribed because difficulties can result from their use at lightly trafficked side roads, where emerging vehicles or turning movements are unexpected; if side road flows are too low then the main road will effectively operate under free flow conditions. Consideration should also be given to the usual site constraints and design criteria. On trunk roads it is unlikely that a miniroundabout would be an acceptable design solution for a new junction.