Evaluating the Highway Capacity Manual's Adjustment Factor for Annual Weekday to Annual Average Daily Traffic: Applying a Consistent Traffic Data Methodology

Document Type

Journal Article

Publication Date


Subject Area

operations - capacity, operations - traffic, policy - congestion


Traffic surveillance, Traffic monitoring, Traffic data, Traffic counts, Traffic congestion, Traffic census, Traffic capacity, New Mexico, Methodology, Methodologies, Highway Capacity Manual 2000, Highway capacity, Gridlock (Traffic), Field measurements, Average annual daily traffic, Annual average daily traffic, Adjustment factors, Accident exposure


The Bernalillo County Public Works Division, New Mexico, developed a methodology to understand and assess traffic monitoring data. The methodology was applied to a national default value used to adjust traffic field measurements. This paper describes research on improving the traffic monitoring process and product. The proposed methodology identifies seven steps to understanding traffic data. The steps can be implemented by an individual analyst, but are recommended for discussion by all stakeholders in the traffic monitoring process including field personnel, office personnel who summarize and report the data, and data users. The methodology was applied to a national default value presented in the 2000 Highway Capacity Manual (HCM) that has been widely implemented, including by Bernalillo County. The factor adjusts traffic summary statistics to represent annual average daily traffic. The factor is used by local governments to adjust short-term traffic counts taken during the workweek so that the summary statistic can be used in a variety of applications including accident exposure rates. Accident exposure rates, for example, are based on traffic for all days, not the workweek. The result of the application of the methodology is that the national default value was found to be inappropriate. Modification to the HCM is recommended. Research to improve further the traffic monitoring process will develop, train, and exercise a team approach to understanding traffic data. Research to further improve the traffic monitoring product will be to identify local data collection sites and compare national, state, and local adjustment factors.