In a downtown area the speeds are likely not high, and the traffic is likely stop and go, with lots of opportunity for revving engines as drivers jam the accelerator down from a dead stop. My previous apartment had a busy highway on one side, and a busy street with a stop light right before an onramp to said highway. The furious acceleration of cars going from a stop to highway speeds as fast as they can was much worse than the monotonous general highway whooshing sounds.