Robert Thorn has worked in the highways for local authorities for 38 years, and prior to his recent retirement, spent eight years testing asset performance on National Highways’ strategic road network (SRN). In this article, he discusses how the local network would have to change to facilitate autonomous vehicles. Driverless car technology may be hard, but maintaining a road it can drive on is not easy either.
Part of the push behind autonomous vehicles is the idea that they could reduce road deaths and casualties, as some suggest around 90% are due to driver error. This is a convincing incentive.
The Government has indicated that approval for these vehicles could be given as soon as 2025. When fully autonomous vehicles are formally permitted on UK roads,
what effect will this have on the standard and maintenance of the network and associated assets? We are already seeing vehicles available with ‘enhanced safety features· such as ‘assisted lane keeping systems· and the capability to ‘read’ road signs and intervene to control the vehicle.
It is worth remembering that National Highways only manages around 2-3% of the road network by length, with the remaining vast majority falling under local authority responsibility. In order to see anything like a 90% reduction in deaths and serious injuries from autonomous vehicles, councils will have to significantly improve their networks. At a minimum this could mean looking into the present and current performance of road markings, traffic signs and street lighting assets and inventory, condition and inspection regimes. It could also mean a shift in policy back to prescribed standards.
Well-Managed Highway Infrastructure: A Code of Practice [2016) allows authorities to operate a risk-based approach to asset standards, maintenance and intervention frequencies with a reliance on visual inspection. I believe this will have to change to more prescriptive standards and testing- akin to the current SRN standards and requirements as detailed in the Design Manual for Roads and Bridges [DMRB). This would help accurately assess renewal schemes and align them with central and local government safety performance and budgetary requirements.
Why must there be prescriptive standards for asset condition?
Consider a scenario where a driver opts for a scenic route in preference to a motorway or dual carriageway. If the network infrastructure is not consistent, the driver may need to take back control.
Things are more straightforward on the SRN because there is a consistency of standards, as National Highways follows those set out within the DMRB. For example, autonomous vehicles might need to read road markings and studs – covered within CS 126, Inspection and assessment of road markings and road studs and BS EN 1436 [Ref 9.N]. They might also need to read traffic signs – covered within CS 125, Inspection of Traffic Signs and BS EN 12899-1.
For the sake of consistency across the entire network, it is feasible that the inspection and testing methodologies outlined in CS 125 and CS 126 for high-speed roads, motorways and dual carriageways could be applied to local roads.
In 2019, the European Road Federation [ERF] published a paper Marking the Way Towards a Safer Future – an ERF Position Paper on how Road Markings can make our Roads Safer, which suggested that road markings should have a minimum standard: ‘Road markings on Europe’s roads should have a minimum width of 150mm for all roads and their performance should not be allowed
to drop below 150 mcd/lux/m’ [R3) in dry weather conditions and 35 mcd/lux/m’ [RW2) in wet and rainy conditions.’
During my discussions with local authority engineers, it seemed most councils identify road marking re-lining schemes through visual inspection, with very few utilising any formal testing frequencies/equipment. Also, most were utilising road materials to R2 100mcd, the next category, R3 150mcd rarely seems to be used. The ERF suggested this would be the minimum performance level so by that standard many local authorities would be required to renew virtually all road markings across their networks. And it would be prudent for authorities to apply material of a higher specification to achieve longer life spans, therefore, if the ERF suggestion of 150mcd minimum is adopted, we could see R4 200mcd being laid, providing additional life span before performance drops to 150mcd.
As a traffic sign fades throughout its life span, the contrast between the legend or diagram and the background decreases and it can become difficult to read, interpret and visualise its retro-reflective characteristics in all conditions. Manufacturers suggest that 10 years after initial installation, signs should be subject to retro-reflectivity testing every two years. As a general guide, once the retro reflectivity of any colour component of the sign has reduced by 20% of its original stated value, an assessment of the sign is required to consider its potential replacement.
In Well-managed Highway Infrastructure, B.5.13.6. there is a specific reference to traffic sign testing: ‘The coefficient of retro-reflection of sign face sheeting is a specialist site test that may require the services of a specialist organisation.
‘Authorities should choose sign performance levels depending on the overall risk assessment and road hierarchy. Highways England [as was] has published TD 25/01 [then TD 25/15 and now CS 1251] which contains more information on the inspection and maintenance of traffic signs.’
Testing of markings and signs
Asset managers working within all levels of the industry will need to utilise regular testing to identify failing markings and signs and to help prepare renewal/replacement schemes. The most safe, efficient, cheap and environmentally-friendly method would be using vehicle mounted data collection equipment, which survey at traffic speed, do not require traffic management and create little disruption to the travelling public This approach is widely used by National Highways.
Technology for testing retro-reflectivity for road markings and traffic signs continues to be developed with some systems already well established. One system used across Europe is a mobile device called AMAC devised by the Cidaut Foundation [a Spanish transport and energy research organisation] which can survey road markings and traffic signs simultaneously at traffic speed, collecting data from a driver’s viewpoint.
In any procurement of independent testing, it is important to cite that equipment and data collection are in accordance with CS 125 and CS 126. Most systems in operation have data outputs in a variety of formats and it is essential that they are compatible with a highway authority’s asset management system.
For road markings, some form of risk-based approach would need to be adopted. Authority engineers will have to consider traffic flows and the hierarchy of the road. Roundabouts, stop lines and safety critical locations invariably suffer high wear rates due to higher braking, acceleration and lane deviation, requiring a specific regime.
Other maintenance issues
On many roads, autonomous vehicles will adopt precisely the same position in the road, equidistant from the centre-line marking and the edge or kerb line. With such a defined route continuously being driven, the levels of carriageway rutting will increase. Also, electric and hybrid vehicles are noticeably heavier than their petrol/diesel counterparts.
Coupled with this, vehicle tyre pressures are higher in an effort to reduce the tyre contact patch area and thereby reduce rolling resistance and hence improve fuel economy. This leads to significantly higher load concentration on road surfaces. Overall, carriageway rutting will become a significant issue for engineers and network operators as a result.
Autonomous vehicles offer considerable safety benefits but would require the road network to reach and maintain uniform standards, presenting new challenges to asset managers that come with a major financial impact. Because of these new standards, authorities will need to make a case to government for extra funding.