Page:UK Traffic Signs Manual - Chapter 8 - Part 1 (Traffic Safety Measures and Signs for Road). Designs 2009.pdf/26

DESIGN CRITERIA cylinders to prevent overtaking at those places where it would be dangerous, while permitting it elsewhere. The appropriate signs to be used are the "road narrows" signs to diagram 516 or 517 with a "Single file trafﬁc" plate to diagram 518, which is generally used only when a lane has been lost. Chapter 4, Section 4 includes general advice on the use of these signs. In no circumstances should a "no overtaking" sign to diagram 632 be used in the absence of a traffic order.

Works in narrow roads may leave insufficient width for public service vehicles to pass. Bus operators and the public transport co-ordinator should be advised of a proposal where:
 * the width of any road which forms part of a bus route is to be reduced to less than 3.0 m; or
 * for two-way working on a bus route, the width is to be reduced to less than 6.75 m; or
 * any bus stop may become unusable during the course of the works.

On all-purpose single and dual carriageway roads, the designer should consider the adequacy of lane width provisions for vehicles to overtake cyclists safely. Department for Transport Traffic Advisory Leaflet 15/99 advises that cyclists need a width of at least 1.25 m to travel safely, thus requiring a minimum lane width of 3.25 m to allow a typical car to safely overtake a cyclist. Where there are HGVs and buses, additional lane width will be required, refer to the Traffic Advisory Leaﬂet 15/99 for further guidance. See also paragraphs D3.32.13 to D3.32.15.

The capacity of a road is defined as the maximum vehicle throughput at a site when there is a queue present upstream. Table 3.2 below gives the likely maximum values for the capacity of normal width traffic lanes in terms of vehicles per lane per hour for different percentages of heavy goods vehicles.

The capacity of narrow lanes should be taken to be 10 to 15 percent less than the figures given in Table 3.2.

At some sites, during the first few days of operation, low traffic capacity may be experienced until drivers become accustomed to the situation. It is important not to make inappropriate changes during this period.

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