The level of hazard for most housing estate elements is not dependent on size – but land use (i.e. roofs, driveways etc). The exception is for roads : low traffic roads are described are low risk and the < 300 movements applies to all use types. Therefore, to be classified as low risk, general access roads should be low traffic with the same level of traffic as expected in cul de sacs and home zones. This would lead to roads for small developments where traffic is less than 300 movements per day on that road being classed as low risk and, for larger developments where more traffic is expected, the risk classification is medium. In a large development, different roads may therefore have a different risk category.
The 300 traffic movements a day suggested as a threshold for moving to a Medium hazard categorisation is only indicative of the type of level at which pollution hazards may increase. It is not based on robust scientific evidence and actual hazard changes will be dependent on a huge range of factors, e.g. the type of vehicles using the road will have a significant influence – for sites with relatively higher proportions of HGVs using the roads, hazards will be greater.
For residential developments, where HGV use is low, and where traffic movements are estimated as around the 300 mark then reasonable judgement needs to be exercised. Normal road use rather than any ‘worst case scenario’ should probably be adopted, but this will depend on the definition of ‘worse case’ and how likely that might be to occur, every day, during the lifetime of the development. If significant infill is anticipated and road use is likely to increase during the design life of the drainage system, then this should also be taken into account.’