Visualising trips and travel characteristics from GPS data

Road & Transport Research, Jun 2003 by Stopher, Peter R, Bullock, Philip, Jiang, Qingjian

The highlighted line (black) in Table 2 shows an increment of 4709 seconds from the previous track point, or an elapsed time of 1 hour 18 minutes and 29 seconds. Clearly, this must be a trip end, occurring between 3:45 pm and 5:03 pm. A further indication that this is a stop is provided by the following rows of data (highlighted in grey), for which the speed is zero for approximately 1 minute, and no distance is covered (i.e., no discernable change in position, as indicated by no change, or very small changes in longitude and latitude values). This is most probably due to the vehicle remaining stationary while the driver gets ready to leave, or waits for a passenger to embark. Such situations make for obvious trip ends. The more difficult ones are very short stops, such as may occur when a person fills up the car with petrol, stops to post a letter, or does some similar activity. Even more difficult to detect are stops where the engine is left running, such as at a drive-through fast food outlet, or to drop off or pick up a person.

To determine how to detect such short stops and stops where the engine is not turned off, a number of experimental runs were done by staff, for which the actual activities along the route were known. From an analysis of these situations, it was determined that the optimal rule would be to select any location where there was a period of more than two minutes, during which speeds of zero were observed on all track points, and where the distance between successive track points was also zero. Although discussed in more detail later, it is worth noting at this point that allowances were also made for the (in)accuracy of the Geologger. This approach is not foolproof. There are instances where traffic stops may exceed two minutes, particularly when a vehicle is attempting to make a right turn at an unsignalised intersection, where the oncoming traffic volume is heavy and traffic is not platooned markedly. Another situation that can be detected by software is one in which there is either no detectable stop, or a very short stop, followed by a 180[degrees] change in heading. To detect the change in heading, it is necessary to look further a field than the immediate two or three track points, because it may take some seconds to reverse a vehicle's direction.

Potential problems: Loss of signal

There are also some potential problems that may arise in the track records. The first of these is signal loss during a trip. This is most often caused by large buildings (or urban canyons), heavy tree canopies, or other problems such as tunnels. The second is the time taken for the device to acquire a signal. From our experiments, we found that the rated time of acquisition of position applies only to a stationary device. The devices in use are rated to acquire signal in 15-45 seconds, and generally succeeded, when stationary, to acquire position within no more than 15-20 seconds. However, if the device is immediately in motion when it is turned on, such as if an in-vehicle device is used and a person gets in the car and drives off immediately, a much longer time may be required to acquire position. This results from the vehicle motion, which requires the device to take longer to fix its position, and may also be exacerbated if there are interruptions to the signal, resulting from tall buildings or heavy tree canopies, while the device is attempting to acquire position. In our experiments, we found that in-motion acquisition time depended on how long it had been since the device was last turned on. For elapsed stop times of less than an hour or two, signal acquisition was still relatively fast and generally about 30-60 seconds. However, if a longer time had elapsed since the last use, the position acquisition could become lengthy, and exceeded, in a few cases, 1 kilometre of travel distance and about 2 minutes or more of time.