Turning Lane

A Turning Lane or Slip Lane is a designated part of a road for the use of traffic making a specific movement at a junction. It can take a great many forms, and at major Grade Separated Junctions it takes the form of a Slip Road.

Junction Approaches

Perhaps the most common turning lanes are those provided at the approaches to give way lines at junctions, whether roundabout or signalised in form. Here a single or dual carriageway road will often widen out to allow a number of lanes for queuing traffic, each lane designated for a specific, or set of movements. For instance, at a signalised crossroads, there could be one lane for left turning traffic, 2 for continuing ahead and one for right turns, the two ahead lanes being narrowed down to a single lane again on the far side with a 'merge in turn' system.

More often, 2 directions are joined, so that two lanes could provide for left or ahead and ahead or right. Where there is little turning traffic, this is sufficient, however if the right turn is particularly busy (for instance) and subject to a filter arrow on the signal sequence, then left or ahead and right would be the options. At really busy motorway interchanges, roundabout approaches can feature as many as 6 lanes, 2 on a dedicated and segregated left turn, and then four leading to the circulatory lanes for different exits.

Acceleration and Deceleration Lanes

These simple turning lanes are provided for left turning traffic at junctions. They give additional space on the width of the through route for traffic turning left in (Deceleration) or left out (Acceleration), and therefore remove some of the dangers of slow moving traffic in the main carriageway. They are commonly found on High Quality Single and Dual carriageway routes, such as the A9 in the Scottish Highlands.

Central Right Turn Lanes

There are two main types of turning lanes found in the centre of a road, that used on a single carriageway and that on a dual carriageway.

Single Carriageways

On a Single Carriageway road, the central right turning lane is generally defined only by paint, although occasionally small traffic islands can feature. The length provided is often defined by the road space available, the volume of turning traffic and the speed of the traffic. Hence, in urban areas, such lanes are often just the length of a car or bus, and form part of a continuous sequence along the centre of the road allowing room for each junction. In rural situations, the road will flare in width to accommodate a central lane (sometimes equally, sometimes on one side only, again depending on the land available) and can provide room for half a dozen or more vehicles, possibly more on the hatched areas if road markings permit.

Traffic islands with bollards are commonly found in urban situations, but are rarer in rural areas, perhaps due to concerns over high speed accident risks. Where two opposing turning lanes are required, for instance at a staggered crossroads, normal practice is to have the stationary traffic facing each other, as this allows a greater length for queuing. However, when the junction layout prevents this, the queuing space can be biased towards the busier turn (for instance a B road over a property access).

Dual Carriageways

Right Turn Lanes on dual carriageways are generally no longer installed due to concerns over accident risks, and those that have been built are slowly being replaced with GSJs where possible. However, a large number still survive on the road network. They obviously include a Central Reservation gap, and normally allow for a lengthy deceleration lane as a third lane for right turning traffic. Almost invariable provision also needs to be made for traffic turning right out of the side road, and so the gap needs to be over 3 vehicles wide, and provide space for traffic to wait in the centre of the dual carriageway before joining it.

In some places, routes which are mainly S2 are widened to D1 to provide a dual carriageway style turning lane on a single carriageway road. This is often an old improvement, such as at White Cross (Brent Knoll), where the road was originally widened to a narrow D2 with additional turning lanes. Safety concerns at the junction saw two of the right turns diverted around the Edithmead Interchange half a mile away, and this has happened elsewhere too.

Slip Roads

A common variation of a simple Turning Lane is a Slip Road. These are found at almost all Grade Separated Junctions, on motorways, dual carriageways and sometimes on single carriageway roads. The principle is the same as a turning lane, whereby a dedicated piece of road is provided for turning traffic to use. Obviously, a slip road is somewhat longer than the examples above, with many needing to change levels between the through route and the roundabout or junction with the minor route.

They are used in their simplest form at Diamond interchanges, where a straight slip lane climbs (or drops) from one route to a give way line on the other. These slip roads start like a deceleration lane, before diverging and running almost parallel to the main route. Obviously the onslip is the reverse. Roundabout interchanges normally see the slip road diverge further from the main carriageway, and more complex interchanges can lead to lengthy snaking sliproads.

Whilst some slip roads are just 100m or so long, others such at the southbound offslip at St Georges Interchange on the M5 extending to ¾ mile. They can even extend to a mile or more on larger interchanges, and the connections between the M5 and M6 average about a mile and a half, although it is debatable whether these are true slip roads, as they form D2M carriageways for much of their length.