 More brazen robbers may resort to throwing rocks, petrol bombs or wet cement at vehicles to bring them to a standstill, or may drop rocks from overpasses.Various debris like concrete slabs, tyres or rocks are also placed on roadways, or a car may be tapped or bumped to induce the driver to leave the vehicle.Preventive police operations on N1, N2, N7, R300, M9 and R102 highways are aimed at reducing crime on these roads, and Durban Metro Police has established a street crime unit that will attend to attacks on motorists in the city.