Flows and capacity

The flow of traffic can be understood as being similar to water running down a pipe. The greater the flow, the larger the pipe needs to be. This is a relatively straightforward concept that suggests the pipe (road) should be big (wide) enough to carry the maximum flow ever likely to occur. However traffic flows vary considerably according to time of day, day of the week, season of the year and weather.

Traffic flow data is often described as an average flow per hour over a 24hour day. Clearly flows will be less at night than during the day. The critical figure to consider is usually the hour of maximum flow (termed peak hour flow) which is usually at about 6.00pm in the evening, with Friday evening usually having the highest flow. The hourly flow at this time is often 10% of the total flow for the whole 24hour day.

There are sophisticated methods to measure traffic flow but a simple way to get a feel of the flow in a particular street is to count the vehicles passing a point at about 6.00pm on a Friday evening. This can be done by videoing the flow of vehicles for five minutes, counting the cars at the playback and multiplying the result by twelve.

The theoretical capacity of a single lane of traffic at 30 mph is 1800 cars an hour.