TRRL LR 626:

The average lead levels found in the centre of the motorway in dry weather in the autumn of 1972 were 20.6 g/m3 for the period 07.00 to 19.00 hours and 15.1 g/m3 for the 24 hour period on weekdays. The driver of a vehicle was found to be exposed to three quarters of the lead levels found in the motorway atmosphere. Lead in the atmosphere and in the surface soil fall off rapidly with distance from the motorway. The medical effects of these pollutant levels are not discussed in this report