Global Positioning System: A useful tool for Intelligent Vehicle-Highway Systems (IVHS)
Travel Time and Delay Studies using GPS
Travel time studies are widely used to document congestion and to quantify the actual impact of highway improvements. Travel time and delay data also provide necessary information for use in route guidance and congestion monitoring systems (Taylor, 1992). Most travel time study techniques involve using probe vehicles. These techniques are conceptually very simple, but their implementation tends to be quite labor intensive. Normally two technicians are required in the vehicle: one of them to drive the vehicle, and the other one to record distance driven and the location and time the vehicle passes predetermined checkpoints. Nowadays, distance-measuring instruments (DMIs) can be used to automatically record distance, time, and speed. However, these units have several disadvantages including a need for frequent calibrations and verification of factors, which have nothing to do with the units (for example, tire pressure), and difficulty in using the resulting data in a GIS environment. Global positioning system (GPS) receivers have the ability to overcome these difficulties and, as a result, they are increasingly being used to conduct travel time studies.
GPS receivers record location in latitude-longitude pairs. However, GPS data files tend to have huge number of records, particularly if data is collected at short time intervals, for example: every one second. As a result, formal procedures for linearly referencing, storing, and retrieving the GPS travel time and speed data efficiently become essential. One way to circumvent the GPS data storage problem involves aggregating the GPS data into highway segments or links so that only segment (or link) travel time and speed data are stored in the database. One of the drawbacks of this approach is that the rich detail of the original data is lost because only segment data are stored in the database. Some of the information contained in the original GPS data includes acceleration and deceleration patterns, control delay, and stopped delay, all of which occur regardless of any highway segmentation scheme considered. In order to access this information it is necessary to store all GPS point data in the database and provide a linear reference to each GPS point before attempting any GPS data aggregation. This referencing can be performed with the help of GIS dynamic segmentation tools. Unfortunately, using this capability has been, until recently, out of the reach for most agencies because of high data storage and processing demands. These limitations are quickly disappearing, though, as more affordable computers with much larger data storage capabilities and faster processors enter the market.
Automatic Vehicle Location (AVL)
AVL is a technologically advanced method of remote vehicle tracking and monitoring using GPS. Each vehicle is equipped with a module that receives signals from a series of satellites, and calculates its current geographical location, speed, and heading. This information can be stored for later retrieval or, frequently, transmitted to a central dispatch/control location where it is displayed on a high-resolution geographical map. Vehicle tracking systems will be useful for the police and emergency response services. The central station usually diverts the vehicle nearest to the site, where the vehicles are required.
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