U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
The collection of traffic speed and volume data was integral to this project because these data provide the before-and-after contrast necessary to assess the effectiveness of the SDCWS.
Pneumatic road tubes and counters were used to collect speed and volume data. The advantage of the road tubes is that they are reasonably accurate, can collect individual vehicle speeds (allowing for spot-checking of the data), are low-cost, and are nondestructive to the existing roadway surface. The counters used were Trax I automatic traffic recorders manufactured by JAMAR Technologies, Inc. The units can collect individual vehicle speeds, headways, vehicle class, and volume.
For each data collection period, the counters were set up to record time, vehicle speed, and vehicle class for individual vehicles. Other metrics such as volume, headway, average speed, etc., can be calculated from these data. Since time on the counter can drift the counters, clocks were checked and reset each time they were used.
Speed and volume data were collected at each test location using the pneumatic road tubes. Data collected about 1 month before installation are referred to as "before" data. Data collected about 1 month after installation are referred to as "1 month after" data. In all States, data will be collected again at about 12 and 18 months after installation (referred to as "1 year" and "18 months" data).
Speed and volume data were collected at three locations per test site. The goal was to understand driver speed selection in advance, at the beginning of the curve, and within the curve. These three locations are described below and shown in figure 15:
In most cases, data were collected for at least 1 day (24 hours) during the week (Monday through Friday). During data collection, the equipment was spot checked to determine whether any problems had occurred. Common problems included the pneumatic tubes getting pulled up from the pavement, the tubes being damaged in some way, or the counters malfunctioning.
Data were checked in the field during data collection to spot problems early, and the full data sets were checked when data collection was complete. Data were checked for the following situations that, based on the team's experience, indicate problems with the counters: