Speed Restraint Within a 20mph Zone
To encourage adherence to the designed maximum speed of 20mph, it is necessary to implement one engineering measure drawn from lists (a) or (b) below in each 20mph zone. In many circumstances, it will also be necessary to implement a complementary measure drawn from list (c).
Measures to reduce visibility for the driver are not acceptable by themselves. Rumble strips are also not an adequate speed-restraint measure. Speed-restraint measures should be located at maximum intervals of 60m, starting within 50m of the entry junction or zone. They must be well-lit and must comply with the Highways (Traffic calming) and Highways (Road Hump) regulations. They may take the form of changes in horizontal alignment, changes in vertical alignment or complementary measures.
The visibility of a route can be agreed on a case-by-case basis, with the standard being applied to the geometry of the road.
(a) Changes in Horizontal Alignment
These should be tighter than the minimum specified for each street type, down to a minimum centreline bend radius of 7.5m. The deflection should be greater than 45 degrees with a mountable shoulder to enable larger vehicles to overrun.
The narrowing of the carriageway to 2.7m for a length not exceeding 7m will cause drivers to wait for oncoming traffic to pass. A 500mm mountable shoulder either side will allow service vehicles to negotiate this obstruction. This measure is not appropriate for shared surfaces.
To be effective, the lateral displacement of the running lane must be at least 2m and the length of the displacement no greater than 10m. A reduction of carriageway width to 2m at the entrance and exit of the chicane is acceptable, but a mountable shoulder may be necessary to provide a 3.1m-wide path for service vehicles. This measure is not appropriate for shared surfaces.
An island should result in a lateral displacement of the running lane of at least 2m. The island may be any shape, subject to the minimum dimensions given below. Mountable shoulders may be used to enable the passage of service vehicles, but no vehicle should be able to overrun the centre of the island. This measure is not appropriate for shared surfaces.
(b) Changes in Vertical Alignment
Round-topped humps should be 75mm high and no longer than 3.7m. They are not appropriate for bus routes, shared surfaces or street types A, B or C.
On street types C and D, which are likely to be used by buses and emergency services, speed cushions should be used instead of humps. They are designed to allow the wheels of buses and wide-wheelbase vehicles to pass either side of the raised area while cars still have to negotiate the hump. They should be constructed in pairs.
Single or successive ramps 75mm in height are appropriate within or at the entrances to shared-surface areas.
A plateau may be created by ramps rising 75mm at a rise of 1-in-12. Unless there is a junction, such a plateau should be no longer than 7m – but where it is to be used by public transport vehicles, it should be 12m in length with a rise of 1-in-15. Tactile surfaces should demarcate the border between carriageway and footway for the benefit of the visually impaired. This may be a good way of slowing traffic for a footpath crossing.
A junction may be treated as a plateau approached by ramps as described in the Speed Tables section of this guide. Tactile surfaces should demarcate the boundary between carriageway and footway.
(c) Complementary Measures
Buildings may form an end-stop to a straight stretch of street, or be angled indicating a change of direction. They may also form a gateway through which the street passes. Used in conjunction with other speed restraints, they can induce drivers to reduce speed and take extra care.
Page updated: 15/02/2018