Sample Size Requirements for Vehicle's Speed Data Collection Using Global Positioning System

Abstract

In recent years, Global Positioning System (GPS) device is widely used for collecting speed, position, travel time and delay data of various vehicle types. GPS device is capable of recording speed data at sub-second accuracy and position data to sub-meter accuracy. In order to have appropriate results, it is necessary to have sample size more than the minimum requirements for various vehicles types. With advent of GPS technology and its widespread use for traffic data collection, it is required to examine sample sizes recommended by Institute of Transportation Engineer's Manual of Transportation Engineering Studies and reported in literature by various other researchers.Reported sample size depends, either on standard deviation of maximum speed attained by vehicle or on speed range of vehicle. These two parameters change with speeding and accelerating capability of vehicle and hence change with vehicle type. This study aims at estimating sample size requirements for GPS speed data of various vehicles types (such as truck, car, motorized three and two wheeler, etc.) plying on Indian roads using various methods reported in literature.In acceleration/deceleration related studies at intersection or at control section where vehicles’ speed varies from zero to maximum (or otherwise), use of standard deviation of running speed (as specified in ITE Manual) or standard deviation of range of speed (Quiroga3) may not be appropriate to determine the sample size for GPS experiments. In such cases, use of standard deviation of free flow speed of test runs are more useful, (Li, S. et al., 2002). Since the standard deviation of free flow speed of vehicles varies with vehicle type (among other factors), the sample size requirements differ with the vehicle type.Experiments are conducted on truck, motorized three wheeler and two wheeler and car for evaluating their sample size requirements using GPS device. Sample size is evaluated on the basis of 5 test runs conducted initially for these vehicle types. Number of initial test runs were decided on the basis of comparison of tα/2 values with Zα/2 values. It is found that after 4 test runs, tα/2 and Zα/2 are equal at 95% level of confidence (Li, S. et al., 2002). Various methods (ITE Manual, 2000, Quiroga, 1998 and Li, S. et al., 2002) are used to decide sample size requirements of different vehicle types. It indicates that ITE manual2 method underestimates the sample size as compared to Quiroga's hybrid method and Li, S.’s modified method. It is also found that Li, S.’s method is best suited to estimate sample size requirement of different vehicle types.