A highway-rail grade crossing is a location where a public highway, road, street or private roadway, including associated sidewalks and pathways, crosses one or more railroad tracks at grade. There are approximately 220,000 railroad crossings in the United States, of which approximately 137,000 cross public roads. A collision between a train and a motorist at a grade crossing occurs approximately every two hours. Half of the crossing accidents happen where there are active warning devices, and more than 60% of accidents occur where train speed is less than 40 mph. Approximately 20% of accidents involve cars running into trains.