Page:UK Traffic Signs Manual - Chapter 8 - Part 1 (Traffic Safety Measures and Signs for Road). Designs 2009.pdf/25

DESIGN CRITERIA The minimum headroom to be provided at temporary structures (for example, scaffolds, falsework, temporary accesses etc.) should always be as follows:


 * where the temporary structure is free-standing the minimum headroom should be as for sign/signal gantries, i.e. 5.41 m;

provide a temporary safety barrier. Guidance on these aspects is given in TD 19 "Requirement for road restraint systems" (DMRB 2.2.8); and
 * where the temporary structure is attached or associated with a permanent structure the minimum headroom for the temporary structure should be the same as for the permanent structure – if the permanent structure has a clearance greater than that given above, the minimum headroom for the temporary structure should be 5.41 m unless it is a high-load route. Designers should consider the potential risk to the temporary structure posed by an errant vehicle and the possible need to


 * where the road has a high-load clearance requirement greater than 5.41 m, the headroom appropriate to the high-load route plus 0.15 m should be provided.

The headroom should always be increased as necessary to allow for vertical curvature, cross fall and the longitudinal grade of the road and take account of the deﬂection of the temporary structure under dead and live loads. If the required headroom is not obtainable over the full width, the part of the carriageway beneath the temporary structure should always be closed. The Highway Authority may use its powers to divert trafﬁc when the required headroom is not available.

On all roads, in order to provide the required lateral clearance, the running lane width may be reduced according to the expected type of usage. Where heavy vehicles, including public service vehicles, caravans etc. are expected, the lane width may be reduced to 3.25 m (desirable minimum) or 3.0 m (absolute minimum). Where two lanes are required for HGVs the near side lane should be 3.25 m (absolute minimum).

On single carriageway roads, two-way operation of trafﬁc should be maintained. This normally requires an unobstructed width of carriageway of 5.5 m, but see paragraph D3.3.6 for considerations relating to bus routes, and paragraph D3.3.7 for considerations relating to cyclists. If the width of unobstructed carriageway is less than 5.5 m, the through passage should be further restricted using cones to ensure that the traffic lane is not greater than 3.7 m and alternate one-way trafﬁc (shuttle working) introduced using one of the methods of traffic control described in Sections D5.6 to D5.8 and D5.10. The minimum width lane at which shuttle working may be maintained is 3.0 m, but, car-only traffic can be maintained with a minimum of 2.5 m. However, where the traffic is expected to consist only of cars and other light vehicles the lane width may be reduced to 2.75 m (desirable minimum) or 2.5 m (absolute minimum). Guidance on the maximum length of works and ﬂow criteria can be found in Sections D5.6 to D5.8 and D5.10. Whenever this situation arises, advance warning of the narrow lanes should be given using signs to diagram 516 or 517.

For works on minor roads (see Section D5.3) where traffic speeds are restricted to 30 mph or less, the width of a single lane may be reduced to an absolute minimum of 2.5 m, for cars and light vehicles. Adequate warning of narrow lanes must be given, for example, by using a sign to diagram 516 and 517; see Section D5.

Works on dual carriageway roads may require some traffic lanes to be reduced in width to less than 3.0 m. Whenever this situation arises, advance warning of the narrow lanes should be given. In these circumstances it will be necessary to re-mark the carriageway; see Section D6.3.

When works are to be undertaken which will restrict the width of a road on which overtaking is normally permissible, it may be advisable to consider the imposition of a "no overtaking" restriction for the duration of the works. However, it is usually possible by means of temporary barriers, fences, cones or 24