D6.3.3 The operation of a layout using narrow lanes, where there is only one lane suitable for wider vehicles (e.g. HGVs and PSVs) in each direction, is limited by the number of such vehicles using the road. Calculations based on local flow characteristics should be carried out to ascertain the ability to carry anticipated wider vehicle flows before utilisation of this method. In the absence of such data, guidance may be found in Section D3.4 and Section D6.12. Where one lane is insufficient to carry the number of wider vehicles using the site, it may be possible to provide a second 3.0 m wide lane if the carriageway will accommodate this, or carriageway widening may be an option. Where two lanes are required to carry the number of wider vehicles expected, it is recommended that the near side lane should be a minimum of 3.25 m wide.
D6.3.4 Where a temporary mandatory speed limit is applied at road works, there may be scope to reduce the set-back and extend the paving to the central reservation or verge, refer also to paragraph D3.2.13. The extent of any reduction in set-back will depend on the requirements of TD 19 “Requirement for road restraint systems” (DMRB 2.2.8) and TD 27 "Cross-sections and headrooms" (DMRB 6.1.2). Where the paving is extended to the central reservation or verge specially to accommodate the reduction in set-back, it should be strong enough to carry the expected traffic load.
D6.3.5 Narrow lanes require a raised level of driver concentration and this should be taken into consideration when determining the maximum length of the scheme. The maximum length of a site is 4 km, except where otherwise agreed with the Highway Authority; see also Section D3.5.
D6.3.6 Ideally there should be no difference in the surface characteristics between any parts of the carriageway. The reconfiguration of the carriageway may place the longitudinal joints or the original road studs within the new lanes and in some cases in line with the vehicle wheel tracks. Road studs must be removed. Where the designer considers that retaining the road stud bases may cause problems to drivers (especially motorcyclists), these should be removed and the carriageway reinstated before introducing traffic to the narrow lane layout. It may require a special traffic arrangement to achieve this.
D6.3.7 The lane delineation markings should be to diagram 1004.1 (6 m mark, 3 m gap) instead of the conventional marking to diagram 1005.1 (2 m mark, 7 m gap) where the permanent speed limit is more than 40 mph. On roads with a permanent speed limit of 40 mph or less, lane delineation markings should be to diagram 1004 (4 m mark, 2 m gap) instead of the conventional marking to diagram 1005 (1 m mark, 5 m gap).
D6.3.8 To avoid driver confusion, before the carriageway is re-marked to the new temporary configuration, it is essential that the original road markings that do not form part of the temporary lane markings are thoroughly removed. The hard shoulder/lane 1 continuous marking need not be removed where it aligns with the temporary lane markings, only the sections at changeover positions should be removed where traffic has to traverse it.
D6.3.9 Contra-flow systems on three-lane carriageways can be extended by completely removing the existing markings from the secondary carriageway and re-marking it into five reduced-width lanes; see Plan DZD5 in Section D6.18. For information on temporary road markings and studs, see Section D3.11. For information regarding minimum acceptable lane widths through the contra-flow section, see Section D3.3. See Section D6.5 regarding the design of crossovers.
D6.3.10 Narrow lane systems tend to even out the disruption to the traffic on the two carriageways. However, it may be necessary to change the lane allocations (i.e. restrict the “primary” traffic to two lanes and allow the full three lanes for the "secondary" traffic) where, say, there is an adverse imbalance in directional flows during peak periods, or because of the presence of slow-moving vehicles travelling up an incline.
D6.4CONTRA-FLOW OPERATION
D6.4.1 Prior agreement of the Highway Authority must be obtained before contra-flow operation may be used. Traffic regulation orders are required to close carriageways and/or install two-way operation.
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