Estimating Traffic Volumes
While use of traffic counters is the preferred method of determining the traffic volume along a given stretch of roadway, it can be time consuming and expensive to take traffic counts along every highway. In many cases, an initial estimate can be used to help determine the relative amount of traffic along your system.
A simple rule of thumb takes advantage of the nature of traffic flow throughout the day. During a typical weekday, fifteen percent (15%) of the traffic occurs during the busiest hour of the day in rural areas. Eleven percent (11%) is the typical value for this busiest hour in urban areas. While the morning and evening rush volumes are not the same, they are similar. If you go out to a location during the morning rush when people are going to work (or in the afternoon when they are coming home) and count for 15 minutes, you can use these figures to estimate the traffic:
- Take a count for 15 minutes.
- Convert this value to a full one-hour count by multiplying by 4.
- Divide this number by the estimated percentage of traffic for the busiest hour: 15% in rural areas and 11% in urban areas. Suburban areas will be somewhere in between, usually close to 12%.
Here is an example for a rural area (15% during busiest hour). Let’s assume one vehicle per minute or 15 vehicles:
- Count for 15 minutes: T=15 vehicles
- Convert to one hour count: Tx4=60
- Divide by 15%: Tx4÷0.15=400