Page:UK Traffic Signs Manual - Chapter 8 - Part 1 (Traffic Safety Measures and Signs for Road). Designs 2009.pdf/33

DESIGN CRITERIA On high-speed roads the greatest required speed reduction applicable from Table 3.5 above should be used throughout the site, also refer to paragraph D3.7.26 above. One exception to this is where the crossover speed limit is lower than at other areas. Another exception is the prevention of over-long lengths of over-restrictive speed restrictions, i.e. where the length of road works is required to exceed 4 km and the speed limit to be imposed on the initial length of road (1 km or greater) is lower than that required for the remaining majority length of road works.

Speed reduction should be considered on a case-by-case basis and should involve a site-specific risk assessment. Generally for motorways and dual carriageway roads normally subject to the national speed limit, a temporary maximum speed limit should not be less than 40 mph and for other high-speed roads not less than 30 mph. Temporary mandatory speed limits are not required for relaxation works.

Applying speed restrictions for factors not listed in Table 3.5, and where drivers are unlikely to identify a potential hazard, is likely to reduce the level of compliance. General safety concerns, such as queuing traffic and inoperable matrix signals, are not normally sufficient to justify speed limits unless there are relevant substandard or non-standard features e.g. insufficient stopping sight distance or active lane control. Where there is sufficient justification for imposing a speed limit in these situations, signs to diagram 7004 should be used to inform drivers of the nature of the hazard, see paragraph D4.13.11.

Where road works are situated near a bend in the road, and especially a left-hand bend, it is essential that adequate advance warning is given to approaching drivers because of the reduced visibility. An indication should be given of the path that must be followed to avoid any obstruction in the carriageway. These remarks apply to all works, including those of a temporary nature, for example, those for which the supplementary plates mentioned in paragraph D4.8.2 below would be appropriate.

A tapered line of cones must start far enough in advance of the obstruction to enable drivers to negotiate both the bend and the obstruction in safety. The tapered line of cones must be located in advance of the bend so that it is clearly visible on the approach side. These precautions will also help to safeguard those persons working on the road.

Where it is considered that positive traffic control is necessary, the requirements set out in Sections D5.8 to D5.10 should be observed.

Sight lines need to be considered from both the traditional highway design perspective where the road users’ needs are considered, and also from the operatives’ point of view (e.g. the design of works access/exit). The project designer should ensure that layouts consider the sight line requirements from both points of view and endeavour to ensure that provision of forward visibility and signing does not compromise the operatives’ safety.

Traffic cones and cylinders are used to delineate the traffic lane a driver should take past an obstruction, accident or road works. The portability of these devices is of particular advantage in emergencies or when they are used to delineate works which move progressively along a carriageway.

Traffic cones should be placed close enough together to give an impression of continuity and an appearance of substance. The size of cone and the rate of taper to be used on different classes of road are specified in Table A1.3 (Appendix 1) which also gives details of the cone spacing depending on their location in a layout and the type of works involved. 32