Quiroga and Bullock (1995) have deduced the following, after performing experiments and collecting over 3 million GPS data points over a network of more than 300 miles. They have shown that to detect localized errors, the segment (the road networks are divided into segments whenever particular attribute changes, one such attribute may be the number of lanes on that segment) lengths of the road network should be around 0.2 to 0.5 miles. A sampling rate of 1 or 2 seconds is preferable and the sampling rate should be smaller than half the shortest travel time associated with the segment. In conducting travel time studies using GPS and GIS, the first step is to obtain a good base vector map with links to a database. It is advisable to construct the base map directly form GPS data, GPS data collected during future travel time studies is guaranteed to match the vector base map with in a tolerance defined by the GPS equipment positional accuracy. The major advantage of using a base map produced by a GPS, against previously existing maps is that the positional errors in the existing maps can be overcome.