The structure of the traffic greatly influences the road run-off pollution load. As a general rule, pollution on low-density roads is smaller than on high-density roads. However, the relation between pollution and traffic density is not linear and it is very difficult to predict the run-off pollution from road traffic characteristics, although these characteristics have a large role in controlling contaminant fluxes. The pollution of road run-off is also highly dependent on the climatic regime.
Traffic characteristics on roads are defined according to the several criteria. The most general parameter is called annual average daily traffic — AADT — however this parameter doesn’t define the structure of the traffic. Usually, this parameter is further defined through the passenger car equivalent — PCE — or by the proportion of the AADT that is heavy commercial vehicles (HCV). These approaches allow consideration of the number of different types of vehicles.
In low volume roads, run-off treatment procedures usually differ from those used with high volume roads. For mechanical reasons, the traffic volume and road design criteria are, of course, connected. Consequently, the pollution potential of all roads is linked not only to the traffic levels, but also to the design criteria and approach adopted to match that traffic level. If the traffic volumes are low, the road’s design is likely to be thin and the attention to detail to handle runoff and seepage waters is likely to be brief or even absent altogether. Thus, while the risks and volumes of contaminants arising may be less than on a heavily trafficked road, there may also be greater opportunity for these pollutants to enter the local water and ground environments.
If there is no exact legislative demand for run-off and seepage water treatment, the designer must define the treatment procedure according to the traffic density forecast and to the structure. Some national legislations (e. g. in Central Europe) attempt to define technical protection measures based on traffic characteristics (e. g. Brencic, 2001). Criteria are then related to AADT and PCE. However, these criteria are not very satisfactory, over-simplifying the situation, and should be reconsidered with new parameters based on potential pollutant equivalents of different types of vehicles being applied instead. In Slovenia, the technical legislation dealing with road run-off pollution was implemented defining classes according to PCE. The legislation defines classes of PCE according to the aquifer types and surface water bodies that are crossed by roads. If the prescribed limit of PCE is exceeded then road run-off treatment should be performed. On the highly vulnerable karstic aquifers the limit is set to 6,000 PCE, on intergranular aquifers the limit is set to 12,000 PCE and on rocks and sediments with low permeability the limit is set to 40,000 PCE.
