- Road categories 0 to 4 are based on the number of millions of standard axles (msa) carried by the road over a 20-year period. Reinstatements are designed on this basis.
Some roads that have been constructed in recent years (particularly new housing estate roads constructed under Section 38 of the Highways Act 1980) may have been designed and constructed to DMRB standards. In these cases, the authority must notify the undertaker, in advance of the works, and the undertaker must reinstate the excavation to match the existing construction thickness. It is not possible to match the engineering integrity of existing well-consolidated unbound layers with unbound reinstatement materials. Therefore, there may be significant differences between the existing carriageway construction and the requirements of the Code. The increased thickness of reinstatement materials compensates for this.
- For any road, its msa rating gives the number of standard axle loads that it is expected to carry over a defined period of time. This msa rating is calculated using the following input data:
- 24-hour annual average daily flow (AADF) of commercial vehicles in one direction. The use of AADF data in one direction, averaged from data in each direction, is recommended.
- Average vehicle axle factor over a 20-year service life.
- Actual sustained annual growth rate of commercial vehicles, averaged over several years, from valid census data.
- Where the actual AADF rates for any road are significantly different for each direction and Table NG1.1 indicates different road types in each direction, the highest traffic category applies in both directions.
- Where one-way traffic systems or other traffic management schemes result in multi-lane traffic, standard growth rate predictions and lane correction procedures may result in an inaccurate road classification overall. In such cases, whenever reasonably practical the flow of commercial vehicles should be monitored separately with traffic calculations completed for each traffic lane.
- Where an existing road is near or beyond its service life and is expected to be re-constructed within the foreseeable future, a temporary re-classification of the road will usually be appropriate pending re-construction. Such temporary re-classifications should be undertaken by agreement between the parties involved. Similarly, where roads are expected to be re-constructed within the guarantee period of the reinstatement, it will usually be appropriate to amend methods, materials or performance requirements for those reinstatements by agreement.
- The national network of roads carrying up to 125 msa within a 20-year period and classified as Types 0 to 4 roads according to the requirements of this Code will yield a distribution similar to that shown in Table NG1.2.
Table NG1.2 Estimated highway classification Road Type % of total 0 < 1 1 < 1 2 < 5 3 < 9 4 > 84
- It is expected that roads in any authority area will show a distribution similar to that shown in Table NG1.2, although there will be some local variations. In future years, there may be cases where traffic flows change to such a degree that re-classification will be necessary.