You found this piece of advice from the Michael Mobbs website, it was compiled by Phil Mulvey, Managing Director, Environmental Earth Sciences (www.environmentalearthsciences.com) about issues of pollution when foraging in urban environments:
Can we eat fruit and produce from roadside trees and plants?
“During the 1990s a lot of research was done on lead emission from car exhausts on Main Roads. It was found that on busy roads lead emissions were limited to 30 cm high and within 15 metres of the edge of the road. There was no noticeable impact on less frequently used non arterial residential streets from car exhausts. The removal of lead from fuel in Australia has resulted in lead no longer being a health issue for emissions. Hydrocarbons and benzene degrade rapidly and do not impact plants, in fact they are a growth stimulant at low levels.
Road verges may historically have elevated metal levels and PAHs levels in soil from runoff of zinc roofing and from fill from unknown sources. This applies to all soil in the inner city area. Plants have protection mechanisms to prevent the uptake of lead. Copper and zinc are trace elements and can be taken up but this is beneficial for humans. It is recommended for all root crops grown in home or public gardens that the roots be washed and peeled before consumption. Furthermore as recommended by the Department of Health all food for consumption should be washed prior to consumption. If this simple common procedure is undertaken health impacts from food grown on roadside verges and other simple public land is not expected to cause any health issues”