Final Report - VHB.com

2009 Design Hour Volume
The unit of measure used to evaluate and design roadway facilities is an hourly
traffic volume measured in vehicles per hour (vph). However, because hourly traffic
volumes can vary during the course of a day and throughout the year, it is necessary
to select an appropriate design hour volume (DHV) condition. The hourly traffic
volume used for the purpose of design should not be exceeded very often or by very
much. However, it should not be so high that the traffic volume would rarely be high
enough to make full use of the facility. It is wasteful to design a facility based on the
maximum peak-hour traffic of the design year, yet the use of the average hourly
traffic may result in an inadequate design. Therefore, the procedure typically used to
evaluate traffic-volume demands on a roadway system, as described in A Policy on
Geometric Design of Highways and Streets, 3 is to establish a 30th-highest hour volume,
or DHV, as the future design condition. Given the economic considerations involved
in the planning and design of roadway facilities, this DHV design criterion is selected
because the 30th-highest hour volume generally reflects a “point of diminishing
return” in that a substantial increase in capacity would only accommodate few
periods of higher traffic volumes.
The MaineDOT generally applies seasonal-adjustment factors to the raw traffic data
to establish the DHV utilizing highway classifications of Groups I, II, and III for state
and local roadways. Group I roadways are defined as urban roadways that
experience little seasonal variation throughout the year because they primarily serve
commuter traffic. Group II roadways are defined as arterial roadways that
3 American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and
Streets, Washington, DC, 2001.
Existing Conditions 29