A traffic signal should not be expected to reduce the total number of crashes at an intersection. Traffic signals in Minnesota, according to a 2013 study, have a statewide crash rate of about five crashes for every 10 million entering vehicles.
An all-way stop has an average crash rate of four crashes for every 10 million entering vehicles, and a two-way stop has an average of three crashes per 10 million entering vehicles. Of course this can depend on other site conditions as well, but on average, a signal will reduce right-angle crashes while causing an increase in rear-end crashes with an overall increase in crashes. Right angle crashes are often the most severe crashes resulting in injury, while rear end crashes involve property damage but are generally less severe.
Unfortunately, drivers occasionally run red lights or fail to notice stopped traffic in front of them. Many of the city's serious and fatal crashes occur at traffic signals. Therefore, it is very important to study an intersection to ensure a traffic signal is appropriate.