Page:UK Traffic Signs Manual - Chapter 8 - Part 1 (Traffic Safety Measures and Signs for Road). Designs 2009.pdf/246

DUAL CARRIAGEWAY ROADS Details of the operation of the Mobile Carriageway Closure technique are given in Part 2: Operations, Section O11. Planning issues, for example, the working windows possible and also consideration of likely queue lengths are considered in Section O11.4.

At sites where there is a marked daily change in the direction of the dominating flow, contra-flow schemes can be modified to permit tidal flow operation. This allows additional lanes to be run in the direction of the higher flow at the expense of the other (less trafficked) direction, reversing as necessary the configuration to accommodate changes in the balance of flows.

During the design stage, it is essential to determine the tidality of the traffic flow, not only to be able to predict the effectiveness of the proposed traffic management system, but also to determine the time when the lanes should be changed.

Consideration needs to be given to the installation procedure. Tidal flow layouts take longer to set out (and to change phases) than conventional contra-flow systems, due in part to the additional buffer zone. Given the amount of equipment to install and the requirement that the layout cannot be left in an unsafe temporary configuration, the feasibility of installation should be addressed at the design stage of the scheme. This would need to take account of the traffic flows and windows of opportunity.

Consideration should be given to the practicability of installing a signing system where the legends of signs that need to be changed can be operated electronically.

Full tidal flow systems on two-lane carriageways are shown in Figure 6.2. Approaching traffic is marshalled into lanes and guided through the contra-flow. Access to, and exit from, the tidal lane is controlled by "gates" at key locations as shown in Figure 6.2 and Plans DTF1, DTF2 and DTF7 in Section D629. Each "gate" should be formed by a line of traffic cones, at 1.5 m centres (Detail B) and, at the gates at which there is a reduction in traffic lanes, there shall also be three "lane closed" barriers to diagram 7105, each surmounted by a "keep left/right" sign to diagram 610.

It is not possible to operate partial contra-flow system on a two-lane carriageway.

Tidal flow systems on three-lane carriageways are shown in Figure 6.4 for a full contra-flow system and in Figure 6.6 for a partial contra-flow arrangement. There is a limitation to the use of full contra-flow systems which arises from the match between lane allocation (1+3 and 2+2) and the peak traffic demand pattern.

In order to provide a second buffer zone to a full contra-flow narrow lane layout (3+2) for a standard carriageway, the lane widths would have to be less than the absolute minimum dimensions given in paragraph D6.3.2 and so this option should not be considered.

A temporary mandatory speed limit will be in place and signs to diagram 670 should be located as indicated in Table 3.4 in Section D3.7. 245