Highways Agency - Safe roads, Reliable journeys, Informed travellers

4. Summary

4.1 In summary:

• Lane use of Lane One by towing vehicles was an average of 84% prior to the pilot signs being installed on 30 July. This increased to 91% during the pilot sign period. This means that one in ten towing vehicles still used Lane Two.
• On average towing vehicles represented less than 2.5% of the total vehicle volume on Fridays 1200-2400hrs, and 4% on Saturdays 0600-1800hrs.
• The pilot signs may have encouraged more towing vehicles to use Lane One, but it did not appear to reduce the incidence of summer weekend traffic congestion on Naish hill.
• The flow of the traffic was often good even with high traffic voume, so the suggestion that higher volumes of towing vehicles (expressed as a percentage of all traffic) cause congestion may be false. The signing pilot did not appear to have an effect on traffic speeds, which would be an indicator of changes in delay.
• One cause of congestion appeared to be cars tending to use Lanes Two and Three, possibly by drivers perceiving these lanes to be less congested when in fact their tendency to use these lanes may actually be the cause of congestion leading to stop-start conditions at the base of the hill; it was observed that stop/start conditions usually started in Lane Three and then migrated to Lane Two and finally Lane One.

4.2 The pilot signing study has shown a change in the proportion of towing vehicles using Lane One in preference to Lane Two, but this has not resulted in the hopedfor reduction in the incidence, duration and extent of holiday period traffic congestion. It is suggested that other factors come into play at this location that affects traffic behaviour. This may include:

• the 'dip' at the foot of Naish Hill caused by the slight downhill gradient approach from Avonmouth Bridge southbound to the start of the uphill section of Naish Hill south of J19;
• cars anticipating Naish Hill and tending towards Lanes Two and Three; and
• cars climbing Naish Hill too close to the preceding vehicle preventing lane changes.

4.3 Observation of the video record suggested that traffic joining the motorway at Junction 19 had no effect on traffic flow and did not exacerbate the congested conditions.

4.4 In the medium term, construction of a climbing lane will assist in alleviating the congestion problems at Naish Hill. Works are programmed to commence on site in September 2005.

4.5 A low cost option to reduce the extent of queue and delay at Naish Hill may be to publicise more widely and with greater clarity that fact that congestion does occur at this location with a reasonable degree of predictability during peak holiday travel periods, and to promote travel during less trafficked hours. Generally the M5 southbound is less congested before midday on Holiday Fridays, and before 0700hrs on Holiday Saturdays. This point was included in the spring/summer 2004 South West Area Roadworks leaflets, but perhaps could be advertised more widely and with greater prominence as part of an information campaign in 2005.

4.6 In addition to the suggested greater publicity, the question of what may be done to reduce congestion during the 2005 holiday season must be considered. Although this would only be in place for a single season before the widening works commence, the potential benefits in terms of journey time savings could be sufficient to justify the cost of implementing a scheme over the summer period, possibly commencing as early as Easter and including the intervening Bank Holiday weekends. Further investigation and consultation will be necessary to determine what specific measures could be included in such a scheme, but it is suggested that measures aimed at limiting lane changes and preventing the bunching observed in lane three are considered. The towing vehicle signing used in the summer 2004 period that is the subject of this report will be considered as part of the overall package.

4.7 It may also be appropriate to consider speed management measures on the M5 southbound between M5 J16 and M5 J20 during holiday periods similar to that used daily on the M25. Whilst the M25 is an expensive and complex scheme to operate, a simplified system may be appropriate under the control of Avon & Somerset Police, perhaps comprising prism variable message signs and a mandatory speed limit of 50mph at peak times.

Appendix A: Highways Agency Press Release