Highways Agency - Safe roads, Reliable journeys, Informed travellers
M5 Junctions 19-20 (Naish Hill) Towing Vehicle Signing Trial
Introduction
The results of this pilot scheme, which was run by the Highways Agency on the M5 motorway near Bristol last summer, are now available. The scheme involved the trial of advisory lane restrictions for towing vehicles on the southbound carriageway at Naish Hill, a two-mile stretch of the M5 immediately south of Junction 19, where severe tailbacks are common during summer weekends.
Read the full report or read on for further background information.
Background Information
The M5 is a vital link to the South West for tourists but long queues regularly develop on bank holiday and summer weekends. These are caused partly by a short steep hill south of Bristol, known locally as Naish Hill, and can extend back beyond the M4/M5 interchange at Almondsbury, a distance of more than ten miles. The Highways Agency has identified that the permanent solution is a climbing lane, for which a contract was awarded earlier this year and is due to be completed by summer 2006.
The Highways Agency monitors traffic flow at this location by permanent traffic counters installed on Avonmouth Bridge. These counters are able to measure traffic flow and speed and distinguish between different vehicle types as well as those vehicles that are towing caravans and trailers. The data shows that the volume of towed vehicles on bank holiday and summer weekends is much higher, as much as four times, and average vehicle speeds are much lower than on normal weekends.
About The Trial
The priority lane trial set out to reduce congestion at Naish Hill near Bristol during the peak holiday period. Drivers towing trailers who switched lanes were seen as making conditions worse. Sudden lane changing causes other drivers to brake and this interrupts the smooth flow of traffic and contributes to the start of the queuing.
Following discussions with motoring and caravan organisations, drivers of towing vehicles were asked not to overtake on Naish Hill at peak periods and advisory lane restrictions were put in place on Friday afternoons and Saturday mornings during August 2004. A press release was issued and widely circulated advising drivers of the restrictions beforehand. Contrary to what was reported in the press, the restriction applied to all towing vehicles and not just to caravans.
Results of the Trial
A report produced evaluating the effectiveness of the trial found as follows:
- On average towing vehicles represented less than 2.5% of the total vehicle volume on Fridays 1200-2400hrs, and 4% on Saturdays 0600-1800hrs.
- Average lane use of the nearside lane by towing vehicles was 84% prior to the pilot signs being installed on 30 July. This increased slightly to 91% following installation of the pilot signs.
- The pilot signs may have encouraged more towing vehicles to use the nearside lane, 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 volume, so the suggestion that higher volumes of towing vehicles (expressed as a percentage of all traffic) cause congestion may be false.
- One cause of congestion appeared to be cars tending to use the two outer lanes, 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 the outside lane and then migrated to the middle lane and finally the nearside lane.
Summary
The pilot signing study increased the proportion of towing vehicles using the nearside lane in preference to the middle lane. The result is encouraging but there are still nearly 1 in 10 vehicles with trailers using the middle lane.
The trial on Naish Hill did not result in the hoped-for reduction in the incidence, duration and extent of holiday period traffic congestion and long tailbacks remain a problem. Study of the closed circuit television recordings made during the trial has shown that a minority of other drivers are not showing proper consideration and have poor lane discipline.
The Highways Agency will continue to study the CCTV recordings made during the trial, which have given us valuable information on driver behaviour and on how tailbacks start, and will now look at how to tackle poor lane discipline by other drivers at this location.
For next summer, the Agency is looking at other measures to improve the conditions and these may include the advisory priority lane restrictions used this summer as part of a package of measures. At the same time work on building a new climbing lane at Naish Hill will continue with construction planned for later in 2005.
Contact Information
If you require any further information about this report or the trial itself, please telephone the Highways Agency Project Sponsor Stephen Coe on 0117 372 8878, or the Highways Agency Information Line on 08457 504030 or via e-mail at ha_info@highways.gsi.gov.uk.
1. Introduction
1.1 This Technical Note has been prepared to detail the results of a pilot towing vehicle signing scheme on the M5 southbound between Junctions 19 and 20, Naish Hill during the summer school holiday period in 2004.
1.2 Naish Hill is immediately south of M5 Junction 19. It forms a continuous gradient of 3% uphill for southbound traffic over a distance of approximately 1.5km / 1 mile.
1.3 Southbound traffic approaches Naish Hill immediately after crossing Avonmouth Bridge. The bridge itself provides four lanes to M5 traffic. The nearside lane forms a lane-drop to M5 J19 for access to Gordano Services, Royal Portbury Docks and the A369 to South Bristol and Portishead. The motorway mainline carriageway continues in three lanes on a slight downhill gradient under M5 Junction 19 before the carriageway starts to rise again on approach to the main uphill gradient of Naish Hill. The southbound on-slip from J19 joins the M5 mainline carriageway just before the start of the uphill gradient.
1.4 During periods of heavy vehicle flow, extensive queues and delays occur at the start of Naish Hill, mainly on holiday weekends, particularly on Friday evenings and Saturday mornings in late July and August, Easter and bank holiday weekends. This congestion frequently extends along the M5 southbound from the M4/M5 interchange at Almondsbury (a distance of 15kms / 9.5 miles) and, at times, as far as Michaelwood Services (30kms / 19 miles).
1.5 It was suggested that congestion occurs at this location as a result of towing vehicles using lanes one and two to climb and overtake on Naish Hill, causing long tailbacks of congested traffic. As a result, a pilot scheme was implemented by the Highways Agency during summer holiday weekends directing towing vehicles to use Lane One on Naish Hill southbound.
1.6 The scheme was discussed ahead of the pilot period with the Caravan Club and motoring organisations and was reported in local and national media. The Highways Agency issued a Press Release on 21 July 2004 (see Appendix A). The press release stated: "On a two-mile stretch of the southbound M5, at Naish Hill immediately south of the Avonmouth Bridge, vehicles with trailers are to be advised to use the inside lane on Fridays and Saturdays from the end of July to help improve traffic flows. The advisory restrictions will be in place between 4.00pm and 8.00pm on Fridays and 8.00am and 2.00pm on Saturdays from 30/31July until 27/28 August."
1.7 The pilot was monitored by video cameras at seven locations for two 12-hour periods: midday to midnight on Fridays; and from 06.00 to 18.00hrs on Saturdays. The period of survey extended from the Friday/Saturday of 16/17 July 2004 to 3/4 September 2004, which included the pilot period 30/31 July to 27/28 August, so that the periods before and after the signing scheme could also be monitored to provide a means of measuring the effectiveness of the pilot.
2. Did towing vehicles occupy Lane Two excessively?
2.1 The starting point of the analysis was to see if the signs directing towing vehicles (which includes caravans) to use Lane One were effective in causing a change in lane use by towing vehicles.
2.2 From the video survey, a count was made of towing vehicles, HGVs and coaches in Lanes One and Two. Overall, in the 12-hour monitoring periods, towing vehicles, HGVs and coaches represented 5.6% of the total flow. Table 1 shows the percentage of the total flow comprising towing vehicles, HGVs and coaches for each 12-hour period of survey.
| % of total flow | Towing vehicles | HGVs | Coaches | Total | Others |
|---|---|---|---|---|---|
| Friday 12-24 | 2.5 | 2.6 | 0.2 | 5.4 | 94.6 |
| Saturday 06-18 | 4.0 | 1.5 | 0.4 | 5.9 | 94.1 |
| Overall | 3.3 | 2.0 | 0.3 | 5.6 | 94.4 |
2.3 Table 2.1 shows that 'other vehicles' represented approximately 94% of all vehicles on Naish Hill during the 12-hour survey periods, most of these being cars. The proportion of coaches expressed as a percentage of the total flow was fairly consistent during the survey period at around 0.3%.
2.4 Towing vehicles represented a greater proportion of traffic volume on Saturday when compared with the Friday survey period - towing vehicles averaged between 1.7% and 2.2% of the 12-hour Friday volume midday to midnight, and between 3.5% and 5.2% on Saturdays 0600-1800hrs. The majority of towing vehicles were cars with a caravan.
2.5 Figure 2.1 shows the percentage of towing vehicles, HGVs and coaches in each lane over the eight week monitoring period. It shows that in the two weeks preceding the pilot, the proportion of towing vehicles in Lane One was between 81% and 87% of the total towing vehicle flow, with the remainder using Lane Two. The average percentage of towing vehicles in Lane One in this period was 84%. This immediately suggests that four in every five towing vehicles used the nearside lane in any case without a need for signing directing them to do so. In the following weeks (from 30 July onwards), the proportion of towing vehicles in Lane One increased to an average of 91% in a range from 88% to 94%. There was no discernible change in lane use by HGVs and coaches. Traffic congestion was still experienced in the pilot signing study period.
2.6 The fact that most towing vehicles were already using Lane One to climb Naish Hill therefore suggests that traffic congestion is not entirely due to towing vehicles using Lane Two. A similar proportion of HGVs also use Lane One, whereas the percentage of coaches in Lane Two is twice as high as that for towing vehicles and HGVs, but the total number of coaches is considerably lower than that of towing vehicles and HGVs.
Figure 2.1 Percentage of All Towing vehicles, HGVs and Coaches using Lanes One and Two
2.7 The change in the proportion of towing vehicles in Lane One from 84% before the pilot to 91% during the pilot indicates that the introduction of signs did have an effect on towing vehicle lane use, but the degree of change is perhaps not significant enough to have had any impact upon the formation of traffic congestion.
2.8 The lane use of towing vehicles was then reviewed in greater detail by comparing lane use in three periods on Fridays and Saturdays, specifically the period before the signs were installed, the period when signs were on display, and the period when the signs were removed. These periods are summarised in Table 2.2 and the percentage of towing vehicles in Lanes One and Two in each period shown in Figure 2.2.
| Time Period and Date. Note that signing scheme only operated in the period 30 July to 28 August 2004. | Friday | Saturday |
|---|---|---|
| Before signs (Time Period 1) | 12:00- 16:00 | 06:00- 08:00 |
| With signs (Time Period 2) | 16:00- 20:00 | 08:00- 14:00 |
| After signs (Time Period 3) | 20:00- 24:00 | 14:00- 18:00 |
2.9 Table 2.2 shows the three time periods considered by the study. Periods 1 and 3 are outside the period when the lane use signs were displayed. These coincide with less heavily trafficked periods, representing the shoulders of the main period of congestion which logically is Period 2 and hence when the signs were displayed. Figure 2.2 shows how the use of Lanes One and Two by towing vehicles compared in each period.
Figure 2.2 Variation in Lane Use by Towing vehicles in the Study Period
2.10 Figure 2.2 shows a degree of variation in lane use by towing vehicles in the two weekends leading up to the pilot signing period, i.e. up to and including 24 July 2004. In the period from 30 July when the signs were displayed in Period 2, lane use is less variable and generally more towing vehicles are using Lane One even when the advisory signs are not displayed. This may be a result of the widespread publicity gained by the scheme in local and national media. In most cases more towing vehicles used Lane One in Period 2 (when the signs were displayed) compared to the preceding Period 1. For the weekend of 3/4 September 2004, the signs were not displayed, but the pattern of lane use is more similar to that from 30 July to 28 August than from 16 to 24 July 2004.
2.11 Overall, it is clear that the signing pilot did encourage more towing vehicles to use Lane One when compared to the preceding periods.
3. What effect did the signing pilot have on traffic congestion?
3.1 Despite the improvement in lane discipline, the pilot scheme had no significant observed effect on the level or duration of the congestion on summer weekends.
3.2 Total traffic volumes during the pilot survey period were comparable to preceding periods, which suggests that the pilot signing did not affect traffic volumes and that traffic flows in the survey period were no bigger or smaller than the shoulder survey periods. In addition, analysis of traffic speed data received from the Highways Agency suggested traffic speeds during the pilot period was not discernibly different when compared to periods before and after. This therefore allows some conclusions to be made of the effectiveness of the signing pilot.
3.3 It is clear from the video survey that the flow of traffic southbound on Naish Hill is often steady even when traffic volume is high, so the suggestion that higher volumes of towing vehicles cause congestion may be false. It was also observed that traffic joining the motorway from M5 Junction 19 had no effect on traffic conditions and did not cause or exacerbate traffic congestion.
3.4 The video survey suggests that it may simply be the volume of traffic, especially cars that are the cause of the congestion on Naish Hill. Cars were observed to drive nose-to-tail not allowing space between them and the preceding vehicle for others to change lanes, and there was a tendency for cars to 'bunch' into Lane Three. A constant display of brake lights was observed during periods of heavy congestion, when cars try to move too fast, causing stop-start conditions in Lanes Two and Three, while traffic flow in Lane One often remained constant and steady. Also the distance between vehicles in Lane One was greater, as cars favoured Lanes Two and Three, which it is suggested are perceived by drivers as operating at a greater speed uphill.
3.5 All these observations are subjective and based on observation of the video survey record.
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.