Motor Vehicle Speed and Volume Amplify One Another as They Increase
The frequency at which a person bicycling is passed by motor vehicles is one of the most useful indicators of the level of stress of a roadway or bike facility. Passing events increase with speed and volume, decreasing rider comfort and safety. Where car traffic is routinely above 20 mph, or where traffic volume is higher than 50 vehicles per direction per hour, pressure on bicyclists from motor vehicles attempting to pass degrades comfort for bicycling and increases risk.
At speeds of 20 mph, streets where daily motor vehicle volume exceeds 1,000 – 2,000 vehicles, frequent passing events make shared roadway riding more stressful and will deter many users.
Between 20 and 25 mph, comfort breaks down more quickly, especially when motor vehicle volume exceeds 1,000 – 1,500 ADT. When motor vehicle speeds routinely exceed 25 mph, shared lane markings and signage are not sufficient to create comfortable bicycling conditions.
Motor vehicle speeds 30 mph or greater reduce safety for all street users and are generally not appropriate in places with human activity.
Where motor vehicle speeds exceed 35 mph, it is usually impossible to provide safe or comfortable bicycle conditions without full bikeway separation.