Highways and Transportation Surfacing Guidelines …

1 Highways and Transportation Surfacing Guidelines minimum PSV of chippings January 2009 In 2000, the then Department of Highways and Transportation produced a policy that modified the requirements of HD 36/99 within the Design Manual for Roads and Bridges (DMRB). It was applied to the design of new wearing courses (now called surface courses) on all highway works. Background Information In specifying appropriate properties of new surface courses, the minimum Polished Stone Valve (PSV) of chippings, or coarse aggregate in unchipped surfaces, is normally determined from Table HD 36/99 of the Highways Agency s Design Manual for Roads and Bridges.

2 Although it is not obligatory for Local Authorities to comply with the standards in DMRB, it is the long standing practice within Leeds City Council, along with most other Local Authorities, to adopt the DMRB as the basis of design standards. The modified policy in 2000 reflected the proven local experience of lower PSV aggregates under certain conditions, it recognized the effect of speed in determining an appropriate PSV, and clarified the approach length over which a specified PSV shall apply. The Highways Agency published a revised HD 36 in 2006 (HD 36/06) and the opportunity has now been taken to re-examine our own modified version and update it in line with the new advice from the Highways Agency.

3 Proposal A revised Leeds Table 1 has been produced and is shown in appendix 1. The new HD 36/06 is quite complicated with ten different traffic categories right up to over 6000 commercial vehicles per lane per day (6000 cv/lane/day). This sort of traffic would only be experienced on the heaviest trafficked motorways and is not applicable to the traffic levels within the Leeds Metropolitan area. The revised table reduces the traffic categories to four. Quite a number of our roads do not have traffic figures (only main roads have regular traffic counts). However, all roads within Leeds have been designated a RASWA* category, which is based on traffic flows.

4 The revised table also shows the four RASWA road categories which can be used when traffic figures are not available. In addition, the range of chippings has been reduced to reflect the type of aggregate available locally. Notes for Guidance also accompany the revised Table 1. * Roads and Street Works Act 1991 Highways and Transportation Surfacing Guidelines minimum PSV of chippings Appendix 1 Table 1: minimum PSV of chippings, or coarse aggregate in unchipped surfaces, for new surface courses RASWA Road Categories Traffic (cv/lane/day) at design life Investigatory level 2 Type 4 Type 3 Type 2 Type 1 Site Category Site Description 1 Speed mph 3 0-100 100-380 380-1000 1001-2200 0-30 55 55 55 55 31-40 55 55 55 55 B Dual carriageways where traffic is generally free flowing >40 55 55 55 55 0-30 55 55 55 55 31-40 55 55 55 60 C Single carriageways (non-event)

5 , where traffic is generally free flowing >40 55 60 65 65 0-30 55 55 55 60 31-40 55 55 60 65 G1/G2 Gradients >50m long >5% >40 55 60 65 68 0-30 65 65 65 68 31-40 65 65 68 68 K Approaches to roundabouts, pedestrian crossings, traffic signals and other high risk areas >40 68 68 HFS4 HFS4 0-30 55 60 60 65 31-40 60 60 65 68 Q Approaches to major and minor junctions, where frequent braking occurs but in a generally straight line >40 60 65 68 68 0-30 55 55 60 65 31-40 60 65 68 68 R Roundabout circulation areas and exit to tangent points >40 68 68 HFS4 HFS4 0-30 55 55 65 65 31-40 60 65 65 65 Bends >100m - 500m >40 65 68 68 68 0-30 60 60 60 65 31-40 65 65 68 68 S1 and S2 Bends <100m >40 68 68 HFS4 HFS4 Notes: Table 1 is derived from HD 36/06 Table Where HFS4 is required, TD 50/04 recommends a minimum treatment length of 50m.

6 However where speeds are over 40 mph a minimum length of 75m is preferred. This may be extended where queuing traffic, speed of traffic, or sightlines indicate that 75m may not be sufficiently long. Where designers are knowledgeable or have experience of particular site conditions, an alternative polished stone valve (psv) can be specified. The actual PSV shall not differ from that in adjacent lanes by more than 5 points. 1 Site categories and descriptions are based on Table HD28/04 2 Investigatory levels based on Table HD28/04 3 Traffic speeds shall be actual, or anticipated, not speed limits ( 85%tile) 4 HFS: High Friction Surfacing to Clause 924 Highways and Transportation Surfacing Guidelines minimum PSV of chippings Notes for Guidance: Investigatory Levels Investigatory levels are based on Table HD28/04.

7 Routine measurements of skid resistance shall be made and processed to derive Characteristic SCRIM Coefficient (CSC) valves. The CSC values shall be compared with the predetermined Investigatory levels. Selection of Investigatory levels are based on Section , IAN 98/07, Guidance for HA Service Providers on Implementing the skid resistance policy. Traffic Figures From the Specification for the Reinstatement of Openings in Highways (New Roads and Street Works Act 1991: Second Edition 2002): Table Road categories Road Category Traffic Capacity Table Table , (6% growth rate) Type 0 > 30 to 125 msa > 2220 to 7350 cv/day Type 1 > 10 and < 30 msa > 1000 and < 2220 cv/day Type 2 > and < 10 msa >380 and < 1000 cv/day Type 3 > and < msa > 100 and < 380 cv/day Type 4 < msa < 100 cv/day The traffic figures in the Specification for the Reinstatement of Openings in Highways are based on 24 hour annual average daily flow (AADF) of commercial vehicles in one direction, whereas the figures in HD36/06 are based on commercial vehicles/day/lane at design life.

8 The vast majority of the roads in Leeds are single carriageways, and where there are dual carriageways the majority of the commercial vehicles will be traveling in lane 1. There is therefore little, if any, difference between the two figures. The figures in the table above are based on an average growth rate of 6%, and the year of traffic count as 2021. This means there is an additional factor of safety built in if the designer has actual traffic counts. Range of Chippings The range of chipping have been reduced (from that in HD36/06) to reflect the type of aggregate available locally. High Friction Surfacing HFS The cost of bauxite, the key aggregate ingredient in HFS rose by 40% between 2000 and 2007.

9 It is likely to rise considerably more in the future due to scarcity of the raw bauxite. HFS should therefore only be used where there are known safety issues. It is therefore recommended that, if there is a known skidding problem, the Surfacing is replaced with the appropriate PSV, rather than simply specifying a HFS.