Noise is easy to dismiss as an annoyance, but the evidence links it to physical harm. Traffic noise at night raises blood pressure in sleeping people, whose bodies react even when they do not wake, and long exposure is associated with heart disease. Children in schools beside busy roads read less well than similar children in quieter buildings. What makes noise difficult to regulate is that no single source is at fault: each vehicle is within the legal limit, and the problem is the number of them.