Page:UK Traffic Signs Manual - Chapter 8 - Part 2- Traffic Safety Measures and Signs for Road Works and Temporary Situations) - Operations 2009.pdf/143



The design process starts with the establishment of the location of the works area and the duration of the working window needed to complete the particular task. This working window should comprise the time required to complete the task plus a safety cushion of two minutes. In general, the working window, including the safety cushion, should not exceed seven minutes.

* Includes an allowance of 2 km for the closure vehicles to move from the near side lane into position across the carriageway.

Table 11.1 shows the distance upstream of the working area that the closure must start in order to provide the required working window, for closure speeds of 20 mph and 30 mph. This distance allows two kilometres for the closure vehicles to get in position across the carriageway and reduce their speed to the planned closure speed. The distances shown assume that the slowest vehicles in the traffic in front of the closure are travelling at a minimum speed of 50 mph.

The presence of any entry slip roads or lane gain/drops on the length of carriageway between the works area and closure start point given by Table 11.1 should then be identified.

It will be necessary to provide advance signing to warn road users of the presence of the slowmoving traffic ahead. Consequently the design process will need to assess the likely queues arising from the operations.

Tables 11.2 and 11.3 provide estimates of queue length for different working windows and flow rates for a closure speed of 20 mph and 30 mph. In general, the mobile carriageway closure operation should not be used for traffic flows exceeding 1200 veh/hr for each lane left open through the works area. The risk to queuing traffic can be reduced by scheduling the closure operation in moderate traffic flow conditions when exceptionally high approach speeds are less likely. 141