Reconsideration of Sample Size Requirements for Field Traffic Data Collection with Global Positioning System Devices

The use of Global Positioning System (GPS) technologies has expanded to perform traffic data collection for transportation studies such as work zone studies. To generate reliable results from the data acquired by using GPS devices, it is necessary to investigate such factors as sample size requirements that may affect a specific study and to establish a consistent method for data collection. It has been confirmed that the Institute of Transportation Engineers’ Manual of Transportation Engineering Studies usually underestimates the sample sizes for travel time and delay studies. However, the hybrid method developed by Quiroga and Darcy overestimates the sample sizes. A modified equation is presented to estimate the minimum sample sizes for collecting field data with GPS devices. Travel speed may be more stable and can be easily measured for travel time and delay studies. Stopped delay varies considerably at intersections, and the sample sizes depend to a large extent on the permitted errors. Work zone layout and construction activities will create variations in vehicle flow within the work zone. To estimate the sample size requirements, it is advisable to use the standard deviation to measure the data dispersion, and a minimum of three initial test runs is required. GPS devices with sufficient accuracy usually require 5 to 10 samples for travel time and delay studies and work zone studies. Stopped delay studies may require a large sample of up to 30 test runs.