Page:UK Traffic Signs Manual - Chapter 8 - Part 2- Traffic Safety Measures and Signs for Road Works and Temporary Situations) - Operations 2009.pdf/104

 The type of vehicle used should be appropriate to the nature of the convoy operation. For instance, small hatchbacks, quad bikes and mini-tractors have been used successfully on shorter single carriageway works whilst medium-sized pickup vans are considered more appropriate on dual carriageways where works are generally longer and intimidation by HGVs is more likely.

Convoy vehicles should be yellow, white or some other conspicuous colour. Vehicles should be kept clean to maintain their conspicuity.

Convoy vehicles should be fitted with roof-mounted flashing amber beacons in accordance with Section O5.3.1.

Convoy vehicles shall display a "CONVOY VEHICLE NO OVERTAKING" sign to diagram 7029, on the rear of each vehicle. Neither the beacons nor the sign should obscure one another. On dual carriageway roads, the “NO OVERTAKING” part of the sign legend may be omitted where the convoy vehicle is unable to turn round, say within half a mile of the end of the works. However, in this case, static mandatory "no overtaking" signs to diagram 632 should be used where this is considered to be a problem, but only where an order prohibiting overtaking is in force.

The number of vehicles required will depend on the traffic flow, the characteristics of the site and the method of employment of the convoy working system. Three-vehicle and single-vehicle systems have both been used with considerable success on single carriageways.

The three-vehicle system is particularly suited to higher traffic flow sites or sites where the convoy vehicle has to turn at a location which is remote from the works, increasing the cycle time of the system and hence the delays to traffic.

When traffic flows are low, a single-vehicle system may be satisfactory. Traffic control in the singlevehicle system should be provided by manually operated "STOP/GO" boards.

On dual carriageway roads, the system does not operate on a shuttle basis. A typical method of convoy working on dual carriageways is described in Section O9.11. The ease with which vehicles can turn and get back to the start of the works therefore influences the number of vehicles required. The empirical formula below may be used to give an indication of the likely requirement on dual carriageway working.

No. of convoy vehicles for dual carriageway =$X⁄Y$+1

Where, X = estimated cycle of convoy vehicle, in minutes (i.e. time to travel through the works + return time).

Y = time, in minutes, for maximum acceptable queue to develop

= $maximum queue⁄vehicles arriving per minute$

Maximum queue is site dependent but is generally of the order of 30 vehicles.

During long-term work, at least one standby convoy vehicle should be provided in case of breakdown to any of the operating vehicles and for use when they are being refuelled. If at any time insufficient vehicles are available to operate the system as planned, convoy operations should be suspended and a full safety zone put in place or the work activity temporarily suspended. 102