Next Generation HICOMP Probe Vehicle Data Collection System

Once a year, the Highway Congestion Monitoring Program (HICOMP) of the California Department of Transportation (Caltrans) issues a comprehensive report on congestion levels for almost 2,300 miles of the state's most heavily traveled freeways. Much of the data is gathered by probe vehicles, which make trips over pre-determined segments and measure the time needed to complete a route. The rest of the data is generated by loop detectors. Currently, the probes use tachometers that register wheel ticks to determine the distance they have traveled. The driver manually notes the start and end times and the route taken, and all the information is stored on a laptop in the vehicle for off-loading and off-line analysis later.

In 2004-2005, CCIT investigated the feasibility and desirability of replacing the current tachometer system with one that uses GPS units to automatically track the location, speed and route of the probe, then integrate the information directly into the PeMS database and generate reports. Over ten GPS models have been tested and the choices have been narrowed down to two systems selected for their ease of use, robustness, and compatibility with existing HICOMP tools. Several Caltrans districts have been interviewed to understand their existing procedure and how they feel it could be improved using GPS.

With good technical results and positive feedback from Caltrans practitioners, CCIT is now developing a prototype that will be delivered by mid-2006. Route and timing information will be provided by GPS. The data collection software will be either off-the-shelf or an upgrade of existing Caltrans software. Additional software developed by CCIT will take the raw GPS latitude and longitude information, collected at 1-second intervals, and tag it with identifiers showing the freeway measured, the direction traveled and the post-miles covered.

The new system will be more efficient and more accurate because:

• GPS measurements provide absolute timing and positioning, suppressing the need to manually input calibration data for probe runs.
  
• The GPS unit can be used in any vehicle, whereas the tachometers are permanently installed in a few selected vehicles.

Access to the data will be easier and faster because:

• Data will be stored in a structured database, eliminating cumbersome file manipulations.
  
• The database will allow easy online access on a continuous basis for performing analysis, and historical data will be available along with the current year’s data.
  
• Ultimately, some of the reporting needs may be generated automatically from the database.