In road safety management, an accident blackspot or black spot is a place where road traffic collisions have historically been concentrated. It may have occurred for a variety of reasons, such as a sharp drop or corner in a straight road, so oncoming traffic is concealed, a hidden junction on a fast road, poor or concealed warning signs at a crossroads.
For some decades treatment of accident blackspots (e.g. by signage, speed restrictions, improving sightlines, straightening bends, or speed cameras) was a mainstay of road safety policy, but current thinking has it that the benefits of these interventions are often overstated. Effects such as regression to the mean,[1] risk compensation[2] and accident migration combine to reduce the overall benefit.
In 1973, Edward Douglas-Scott-Montagu, 3rd Baron Montagu of Beaulieu said in a debate in the House of Lords:[3]
In some cases it has been claimed that the end result of such interventions in accident blackspot areas is an increase in overall casualties. In one notable experiment, a number of accident blackspots were "treated" with a null treatment—placement of a garden gnome, according to some reports. Crash rates at these points were found to have decreased significantly in the following period, a finding which is taken as clear evidence supporting the theory of regression to the mean.