Freeway Traffic Flow Rate Measurement: Investigation into Impact of Measurement Time Interval

Traffic flow rate, the equivalent hourly rate of vehicles passing a point during a given time interval, is a fundamental measure of traffic status that plays a critical role in many aspects of transportation engineering. The time interval used in measuring the flow rate, defined as the measurement interval, is influential in the characteristics of flow rate measures. In fact, the Highway Capacity Manual recommends that transportation engineers use a minimum of 15-min measurement intervals in order to ensure “stable” traffic flow rates. In addition, numerous researchers have commented about the lack of stability of flow rates resulting from short measurement intervals. However, there is not a quantitative understanding of the impacts of short (less than 15 min) measurement intervals on traffic flow rate. This paper presents the results of a study using nearly 30,000 flow rate measurements collected at multiple freeway locations to quantify the impact of measurement interval. It was found that stable freeway flow rates may be calculated using measurement intervals as short as 10 min, and that statistically significant improvements in stability can be achieved by adding 2 min to any measurement interval.