D6.4.14 Opposing flows may be separated by a buffer zone, a buffer lane or a vehicle restraint safety barrier. Where opposing lanes of traffic are separated by a buffer zone (see paragraph D6.4.2), signs to diagram 7201.1 variant, 7203, 7203.1, or their variants, should be provided at ½ mile intervals to indicate the existence of the adjacent opposing lane. Signs to diagram 7201.1 variant are used where there are lane width restrictions and the opposing lanes of traffic are indicated by a single reverse white arrow with black outline, symbol diagram 7288. The bottom panel to diagram 7201.1 variant and the supplementary plate to diagram 7209 for signs to diagrams 7203, 7203.1, and variants, are used to indicate the distance over which the contra-flow condition applies. Signs to diagram 7203.1 and variants are used to indicate where a hard shoulder is used as a running lane and the hard shoulder/lane 1 continuous road marking remains in place.
D6.4.15 Where opposing lanes of traffic are separated by a buffer lane or by a temporary vehicle restraint safety barrier (see paragraph D6.42 and Plan DZD3 in Section D6.18) signs to diagrams 7204/7205 must be used instead of 7203/7203.1.
D6.4.16 The overall width of the contra-flow buffer zone may be reduced from 1.2m to 0.9m but 1.2 m should be used where width permits and desirable minimum lane widths have been accommodated, see paragraph D6.3.2.
D6.4.17 Partial contra-flow is not normally operated on two-lane carriageway roads because there is insufficient width to operate three running lanes.
D6.4.18 Subject to a site specific risk assessment, a relaxation case for a single lane crossover is permitted (Plan DZC6) which enables part-time or short-term contra-flow operation. Following a risk assessment, any existing road markings and studs that traffic overruns in the crossover lane may be retained.
D6.5CROSSOVERS
D6.5.1 It is essential that the transition or temporary crossover is designed to an acceptable design speed and mandatory or advisory speed restrictions set accordingly. TA 92 "Crossover and changeover design" (DMRB 8.4.6) provides advice on crossover and changeover design. Alternatively Tables 6.1] to 6.4 below provide crossover lengths for some commonly used layouts, see paragraph D6.5.7. A smooth ’S’ shaped curve to suit the site conditions should be used. It may be necessary to change the layout of safety fencing and provision of lighting to accord with the design of the adjacent section of carriageway. The design of crossovers should take account of the following:
- curvature of swept paths and vertical alignment considerations — it is essential that radii and crossfalls are consistent with anticipated speeds and speed limits,
- position in relation to existing road layout, street furniture, gantries and lighting columns,
- surface condition and skid resistance,
- removal or covering of existing road markings where appropriate;
- required construction depth in relation to existing central reservation construction,
- blanking off bases from removed lighting columns, safety fence post sockets and illuminated signs,
- severance of existing safety barrier and space required for temporary anchorages — terminal ends should be anchored in accordance with TD 19 "Requirement for road restraint systems" (DMRB 2.2.8) — attempting to secure dropped ends of safety fence lengths with sandbags is not acceptable and is likely to cause serious injury in the event of vehicle impact;
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