Emerging Issues in the Use of Gps for Travel Time Data Collection

Travel time studies are widely used to document congestion and quantify the impact of highway improvements after construction has been finished. For safety reasons, these studies are typically performed with two people in the automobile. The driver is responsible for “floating” with the traffic stream. The passenger is responsible for recording the time the vehicle passes predetermined landmarks. Occasionally, a device that plugs into the odometer is used to log speed between landmarks. This data collection methodology works quite well provided the driver and data collector are conscientious and consistent in their data collection. However, since data collection personnel typically are not highly trained, it is difficult to maintain consistency when a vehicle passes, say, an entrance ramp or exit ramp. Odometer devices help some, but must be carefully initialized and calibrated between vehicles. Due to production economies of scale, it is now possible to purchase a Global Positioning System (GPS) receiver and a notebook computer for $2,000 (Advanstar 1996) to record the velocity and location of a probe vehicle every one second. This has two significant benefits: 1) There is no need for a passenger to record data during travel time runs. 2) There is no need to rely on technicians accurately or consistently recording the location of landmarks since the GPS receiver provides absolute locations every 1 second. However, since a one hour travel time run would collect 3,600 position and velocity records, the data recorded from the GPS receiver is too voluminous to be directly useful. To reduce the GPS data to a meaningful tabulation, it must be overlaid on a highway map and the travel time between particular locations determined. This paper describes procedures for: importing GPS data into a Geographic Information System (GIS); obtaining travel time along segments of a highway network; and developing report templates used to document travel time on color coded maps and 11x17” report pages. DATA COLLECTION METHODOLOGY To use GPS and GIS technology for conducting travel time studies it was essential to construct a base vector map that could be linked to a database. One way of obtaining this base vector map is to use digitized quad maps or TIGER files. However, both these sources do not have the required accuracy. Instead, study personnel drove probe vehicles over all study routes in both directions collecting GPS data every one second. During this field work they also drove all 1 Submitted for consideration to the National Traffic Data Acquisition Conference, May 6-9, 1996. 2 Assistant Professor, Department of Civil Engineering, Louisiana State University, Baton Rouge, LA 70803; 504/388-5246; FAX: 504/388-5263; e-mail: [email protected]. 3 Research Associate, Remote Sensing and Image Processing Laboratory, Louisiana State University, Baton Rouge, LA 70803; 504/388-5260; FAX: 504/388-5263; e-mail: [email protected]. 4 Research Assistant, Remote Sensing and Image Processing Laboratory, Louisiana State University, Baton Rouge, LA 70803; 504/388-5270; FAX: 504/388-5263; e-mail: [email protected]