It is shown that quietening heavy vehicles brings an appreciable reduction of traffic noise for streams containing 20 percent or more heavy vehicles and is most effective for high flows. Conversely reducing light vehicle noise brings most benefit in low flow situations where there is less than 20 percent heavy lorries. For all traffic containing up to 40 percent heavy vehicles reducing the noise from both categories gives an appreciable reduction over and above that obtained by reducing the noise of either category alone. Thus there is a strong case pursuing the development of quieter light vehicles with the same urgency that is at present being given to the development of quiet heavy vehicles. The rate at which quiet vehicles are introduced will significantly affect the rate at which traffic noise levels are reduced. For the heavy vehicle class it is estimated there will be but little benefit until at least 50 percent of the heavy vehicles on the road are quiet vehicles