Reconsideration of Sample Size Requirements for Field Traffic Data Collection Using GPS Devices

In recent years, the use of 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 using GPS devices, it is necessary to investigate the factors such as sample size requirements that may have an impact on a specific study and to establish a consistent method for data collection. Based on the discussions and the real life GPS data presented in this paper, it is 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 et al. overestimates the sample sizes. A modified equation is presented to estimate the minimum sample sizes for field data collection using 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. For GPS devices with sufficient accuracy, it usually requires five to ten samples for travel-time and delay studies and work zone studies. For stopped delay studies, it may require a large sample of up to 30 test runs.