Traffic volume counts are basic to all phases of highway development and operation. No other single reference tells an engineer as much about a road as the number of vehicles which use it.
Traffic volumes are needed for street and highway project developments, financing considerations, project cost-benefit comparisons, project priority determinations, analyzing, monitoring and controlling traffic movement on roadways, traffic accident statistics, research purposes, street and highway maintenance, public information, highway legislation and other public and private purposes.
Traffic volumes vary from place to place, even along the same highway or roadway segment. Traffic volumes also vary from hour to hour, day to day, month to month and year to year. Both location and time elements must be properly identified and related to one another to develop accurate traffic volume data.
Traffic counts are the major source of traffic data. Traffic counts are very specific in that they only apply to one location and to the time period for which they have been obtained. Some of the major types of traffic counts in general use by engineers are annual counts, peak hour counts, turning movement counts and classification counts.
Annual counts refer to traffic volume counts that are taken over a period of days throughout the year and converted to a single number known to engineers as Average Annual Daily Traffic (AADT). This number is reasonably close to the traffic volume that one could expect to see on any given day of the year. These volume counts are use for a number of engineering, economic and public purposes:
As a yardstick for evaluating present highway problems
As a criterion for safety evaluation
As a basis for planning and design estimates
As a tool for establishing need priorities
As a reference for public information purposes
As a reference for other traffic volume computations
Peak hour counts are traffic volume counts taken during the time period of the day most likely to produce the highest volumes during any particular 24-hour period. For instance, the most common peak hour counts of interest to engineers are those that occur in the morning and afternoon. These usually occur around the times that most people are traveling to and from work; however, there are times when the peaks occur at less obvious times. These peaks may be due to a large employer having a staggered starting or quitting time, a school or college, or some other out-of-the-ordinary occurrence. The traffic engineer needs to have this information to properly evaluate the impact of this traffic pattern on the roadway network. Among the uses for this type of volume count are:
Turning movement counts are taken at intersections to determine the actual movement of traffic through the intersection. Traffic engineers and others have a number of uses for these counts:
As a tool for roadway planning and alignment studies
As a tool for intersection design
As a tool for traffic signal system design
As a tool for evaluating traffic volume impacts
Classification counts are just a little different from simple traffic volume counts. In addition to determining the numbers of vehicles passing a given point on the roadway, classification counts also separate the traffic stream into its vehicle-type components and speed components; that is, how many passenger cars, how many trucks, how many vehicles pulling trailers, etc., as well as the variations in speed of the traffic stream. As one might very well imagine, this data is very important to engineers for a variety of reasons:
As a means of determining percentages of trucks, buses, etc. with respect to the overall traffic stream
As a tool in neighborhood traffic calming studies with respect to "cut-through" traffic
As an aid in speed studies
As a tool in determining the appropriate use of traffic control devices