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17 ignore bus movements in layover areas that occur more than this research concerns the data channels. Because there may be 3 min before scheduled departure time, because bus move- hundreds of buses per radio channel, the radio traffic has to be ments in staging areas were being falsely detected as trip starts. managed to fit within the available capacity. Staff at several agencies recommend locating signposts (real or virtual) not at route terminals, but a few minutes' travel from Polling Records the terminal. This arrangement resolves some of the match- ing issues but leaves running time and schedule adherence at Most real-time AVL systems use round-robin polling to the route endpoints unmeasured. (Stop-level data collection track their vehicles. The polling interval depends on the num- effectively accomplishes the same thing.) Records made dur- ber of vehicles being tracked per radio channel; 40 to 120 s is ing the layover, and algorithms using those clues can help typical. Within each polling cycle, every vehicle is polled in reduce end-of-line matching errors. turn, and the vehicle responds with a message in a standard End-of-line complications also affect passenger count format. Round-robin polling is an effective protocol for avoid- analysis, as discussed in Chapter 8. ing message collisions; however, the need to transmit messages in both directions, with a time lag at either end for processing and responding, means that a significant amount of time-- 2.3 Data Recording: on the order of 0.5 s--is needed to poll each bus. The polling On- or Off-Vehicle cycle is therefore limited by the number of buses being One essential distinction within automatic data collection monitored per radio channel. systems is whether data is recorded in an on-board computer A polling message includes ID codes (for the vehicle, its run or in an off-vehicle central computer to which messages are or block, and perhaps its route) and various fields for location sent via radio. Historically, AVL systems have been tied to the data. Location fields depend on the location system used. For radio system and have used that connection for off-vehicle a beacon-based system, they include ID of the most recently data recording. Stand-alone APC systems, in contrast, record passed beacon and odometer reading. For GPS systems, GPS data on board. From the viewpoint of a data archive, radio- coordinates will be sent, perhaps with odometer reading as based systems are limited by radio channel capacity, while on- well. The polling message often includes other fields such as board storage imposes no meaningful limit on either the an operator-activated silent alarm and mechanical alarms number or detail of data records. such as "engine overheated." Until the mid-1990s, on-board data storage was expensive Polling provides location-at-time data (i.e., the location of and was therefore avoided. Since then, the cost of adding stor- the bus at the arbitrary time at which it is polled). However, the age capacity to on-board computers has ceased to be a signifi- much more useful time-at-location data (i.e., time at which a cant factor in system design. Therefore, newer systems, which bus passes a point of interest such as a stop or timepoint) is the sometimes blur the traditional AVL-APC distinction, can be format needed to analyze schedule adherence and running designed with either on- or off-vehicle data recording, or both. time. While polling data can be interpolated to get estimates of In the mid-1990s, Tri-Met made the significant step of specify- time-at-location data, such interpolation can involve signifi- ing that its new AVL system both transmit messages by radio to cant approximation error, especially when buses are traveling serve real-time applications and store event records on board at low speeds because of traffic or stopping. In principle, if for off-line analyses. Because of radio channel limitations, the polling interval were very short, approximations would radio messages are sent only for exceptions (a bus is more than become insignificant; however, radio channel capacity limita- 3 min late or off route, or an event of interest occurs); on-board tions make short polling intervals impractical. (Many systems recording, in contrast, has no such limitation. can switch to a short polling cycle for particular buses in an emergency, but that can only be done to the detriment of other buses' polling interval.) 2.3.1 Radio Messages and Record Types The researchers found no examples of transit agencies Radio systems use a wide area network (WAN) to manage extracting time-at-location information from polling data or communication between a central computer and on-board basing any analysis of running time or schedule adherence on computers and radios. They are licensed by the Federal Com- it. The only off-line use found for polling data was for detailed munications Commission (FCC) and have a limited number of investigations of incidents using playback. radio channels. Limitations are most severe in large cities, where the demand for radio channels (for police, fire, taxi fleets, etc.) Event Records is greatest; unfortunately, large cities also have the larger bus fleets and therefore need more radio channels. Transit agencies In addition to round-robin polling, WANs also support typically dedicate some channels to voice and others to data; messages initiated at the vehicle, generically called "event
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18 messages." Each event record has a code and specified format. board computer, uploading them overnight. However, some Modern AVL systems can have 100 or more different types of systems, including Metro Transit's, find enough radio channel event records. capacity to send stop messages over the air, though only for the Messages initiated by on-board computers are likely to subset of the fleet (under 20%) instrumented with APCs. collide--that is, one bus will try to send a message while the The data items typically included in a stop record--in channel is busy with another message. WANs manage this addition to the usual time stamp, location stamp, vehicle IDs, kind of network traffic problem in various ways, such as by and door switches--are door opening and closing times and having messages automatically re-sent until a receipt message (if available) on and off counts. If routes are tracked by the is received. This need to manage traffic limits the practical on-board computer, as is the case with stop announcement capacity of radio-based communication, because, with ran- systems, the stop record will include stop ID in addition to domly arising messages, the channel has to be unoccupied a generic location information; otherwise, the data is matched relatively high fraction of the time (unlike with round-robin in later processing. polling) to provide an acceptable level of service. In the face of limited channel capacity, then, radio-based systems have to Other Event Records be designed in a way that limits the frequency and length of messages sent. Timepoint and stop events are the frequent events that most off-line analysis relies on. Other event records can be valuable, either in their own right or because of the clues Timepoint Records they offer for matching. In most AVL systems, the timepoint event, indicating a bus's Events whose records can be helpful for matching include arrival or departure from a timepoint, is the most frequent the operator signing in, bus passing a beacon, bus going off event record used for archived data analysis. The event can be route, and engine being turned off or on."Idle" event records, defined in various ways, depending on the location system. indicating that a bus has not moved for a certain amount of Where GPS is used and door switches are not, it is common to time, are useful for confirming bus behavior at layover points report when the bus first reaches a circular zone (typically a or timepoints. Heartbeat records, written every 30 to 120 min, 10-m radius) around the stop. The timepoint record may also help confirm that the bus's data recording system is working. include the time the bus left that zone. In principle, timepoint Events of direct interest for off-line analysis include wheel- records could also include fields indicating when doors first chair boardings or alightings, bicycle mounting, various delay opened and last closed; however, the researchers are not aware types (e.g., drawbridge, railroad crossing), and pass-ups. Most of any radio-based systems incorporating door information. such event messages are manually triggered by the operator; The level of detail of timepoint records affects their accuracy although some (e.g., wheelchair lift use) can be automatically and value for off-line analysis. For example, some running time generated. and schedule adherence measures are defined in terms of departure times, others in terms of arrival times, and others 2.3.2 On-Board Data Recording involve a difference between arrival time at one point and and More Message Types departure time at a previous timepoint. Off-line analysis there- fore benefits from having both arrival and departure times In contrast to radio-based data recording, on-board data recorded, particularly if operators hold at timepoints. Records recording essentially offers unlimited capacity (because on- of when buses enter and depart a stop zone are only approxi- board data storage is so inexpensive it is easily obtainable). mations of when buses arrive and depart the stop itself. Errors It is also more robust, not being subject to radio system fail- can be significant in congested areas where traffic blocks buses ure. (Some radio-based systems, such as Metro Transit's, from reaching or pulling out of a stop. Detail on door opening include backup on-board data storage during periods of and closing, and on when the wheels stop and start rolling, can radio failure to prevent data loss. Event records stored on board help resolve ambiguities and make arrival and departure time are uploaded at pull-in using the radio system and merged into determination more accurate. the event database.) Stop Records Radio Integration and Operator-Initiated Data Stop events are much more frequent than timepoint events, Expecting operators to key in event data that is used for and therefore, far more demanding of radio channel capacity data archiving only is considered unrealistic. Therefore, event if transmitted over the air. Therefore, most AVL-APC systems records are generally limited to what can be automatically collecting data at the stop-level store stop records in the on- generated, unless the on-board computer is connected to the