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55 acceptance tests, as the vendor does in research and develop- trip has 2 excess ons. After five trips, load on every segment ment. When the CTA tested its new APCs, it enhanced the will appear to be 10 passengers greater than it actually is. reliability of its manual counts by using a three-person crew APC processing software controls drift by regularly resetting staffed by managers and data analysts who had a stake in the the system state (i.e., load on the bus) at points of known load. outcome and, therefore, reason to be meticulous in counting On most transit routes, these points are terminals or lay- accurately. over points at which, either by custom or operating rule, the through-passenger load is zero. Blocks are then parsed at known-load points into sections that may be called "sub- 8.1.2 Block-Level Screening blocks," which are usually single trips or round trips, with load Because APCs count both ons and offs, large errors (e.g., due at the start and end of each sub-block set to zero. to malfunctioning equipment) are easy to spot based on a Data structures and processing software need to account large difference between total on and off counts over a bus's for some trip ends being points of zero load and others not; daily duty ("block"). Screening criteria vary. Tri-Met rejects moreover, there may be points of known zero load that are blocks whose on and off totals differ by more than 10%; King not trip ends. County Metro requires that a block's total offs be no more than 7% below or 15% above the block's on total. (The asym- 8.2.1 End-of-Line Identification metric criterion is a concession to recognized counting bias and Activity Attribution with its older APC system.) The earlier review of automatic location measurement pointed out difficulties often encountered in correctly iden- 8.1.3 Accuracy of Load and tifying the end of the line. This problem is most severe in Passenger-Miles Measurements older APC systems that lack sign-in data and suffer from low Because the accuracy of load and passenger-miles meas- schedule-matching rates. urements depends not only on raw count accuracy, but also When the general location of a route endpoint can be cor- on the processing system's ability to parse blocks into trips rectly matched, the end-of-line arrival and departure time and deal with on-off imbalances, accuracy of load and related issues that vex running time analysis are not a concern for pas- measures is a good system test and deserves to be examined senger counts; rather, the main challenge is attributing ons and in its own right. STM sets a good example, requiring that the offs to the right trip. At a simple route terminal, the usual pro- average absolute error in departing load be less than 5%. cedure is to attribute alighting passengers to the arriving trip Relative errors in load can be much greater than those of on and boarding passengers to the departing trip. Sometimes, the and off counts. For example, Kimpel et al. found that, while sys- boarding and alighting activity are sufficiently simultaneous tematic error for Tri-Met's on and off counts were below 2%, it that APCs make a single stop record, which processing soft- was 6% for departing load (37). Because passenger-miles are a ware has to split. Sometimes many stop records will be gener- weighted accumulation of loads, one can expect its bias to be ated at the terminal (and at other stops as well) as the bus may similar to that of load. go through several cycles of opening doors to let passengers One cause of load errors can be the balancing method board, then closing doors to preserve or keep out the heat. used. Kimpel et al. determined load using their own trip-level On-board APC analyzers vary in their ability to take external balancing method, because the block-level balancing algo- inputs (e.g., odometer pulses) and use them in determining rithm used by Tri-Met yielded loads with greater bias. Block- when to close a stop event and start a new one. Off-line pro- level balancing biases loads upward because upward errors cessing software has to have the flexibility to recognize and are permitted to propagate through the day, while propaga- handle both single- and multiple-record cases. tion of downward errors is limited because of a restriction against negative departing loads. 8.2.2 Inherited and Bequeathed Passengers Operating practices for some routes allow passengers to 8.2 Trip-Level Parsing remain on board at the end of one trip in order to ride on the Screening based on on-off balance protects on and off next trip. One example is a route that ends with a loop; totals from substantial errors. However, because of a phe- another example is a pair of interlined routes for which nom- nomenon called drift, substantial errors can still develop in inally transferring passengers actually remain on board. Data calculated load (accumulated ons minus accumulated offs) structures have to identify which route ends are not necessar- and passenger-miles (a weighted sum of segment loads), even ily zero-load points and recognize passengers inherited from with small errors in raw counts. To illustrate, suppose each a previous trip.

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56 To permit trip-level data analysis, the most direct way to this point is treated as passengers inherited by the trip leaving deal with inherited passengers is for databases to include an the loop. extra stop record at the start and end of each trip indicating the number of passengers inherited from the previous trip The Overlapping Loop Model for Short Loops and bequeathed (left on board) to the next trip. Manual ride check forms often start and end with a row for passengers left A third approach, used by NJ Transit on its Philadelphia on board. routes (38), is to model a bus in a loop as serving two trips at An alternative arrangement, proposed by at least one ven- once, attributing alightings to the trip entering the loop and dor, makes no record of inherited passengers but assumes boardings to the trip exiting the loop. Figure 19 illustrates the that any imbalance in ons and offs at the trip level can be overlapping loop model. With this model, boardings and explained as inherited or bequeathed passengers. If a trip alightings occurring within a loop are attributed to the trip has more offs than ons, the difference is treated as passen- that "naturally" serves those passengers; there are no explicit gers inherited at the start of the trip; if more ons than offs, inherited passengers, avoiding messy apparent transfers. This the difference is treated as bequeathed passengers at the end model is also well suited to making balancing corrections. of the trip. However, this shortcut has several shortcomings. The overlapping loop model relies on two assumptions Imbalance can be due to counting errors as well as inherited about passenger travel: passengers. In the face of counting errors, this approach will violate the law of conservation, because it does not guaran- General Loop Assumption: No passenger rides around the tee that the number of passenger bequeathed by one trip entire loop. equals the number inherited by the next. By forcing correc- Short Loop Assumption: No passenger's trip lies entirely tions to be positive (i.e., as opposed to correcting imbalance within the loop. by reducing ons or offs), it biases upward the number of pas- sengers; and by concentrating the corrections at the route Discretion is needed in defining the boundary between ends, it increases average passenger trip length. Together, the entering and exiting trip for boardings. For NJ Transit these factors combine to bias passenger-miles upward. Also, routes into Philadelphia, boardings on the exiting trip this approach cannot resolve imbalances on routes with clearly begin at the first Philadelphia stop. However, on loops at both ends. some routes with loops, the exiting trip's boardings may begin at the last trunk stop (Stop A in Figure 19) if travel time around the loop is smaller than the service headway, in 8.2.3 Routes Ending in Loops which case passengers waiting at A' have nothing to lose by Many transit routes end in loops that lack a natural termi- boarding at A and circling the loop. nal point at which buses always empty out. Examples are With the overlapping route model, a short loop effectively radial routes with a loop at the suburban end for wider cov- serves as a fixed-load point for parsing, screening, and on-off erage and commuter routes with a collection/distribution loop balancing. For the entering trip, load is fixed at zero when it through the downtown, such as NJ Transit routes into has finished serving the loop; for the exiting trip, load is fixed Philadelphia. There are three ways to deal with attributing at zero as it begins serving the loop. passengers who board or alight on these loops; each approach An example of fixing and balancing load on a loop with has important implications for data structures. four stops (A through D) is shown in Table 9. In Table 9(a), there is an imbalance of 2 excess offs; however, after the data is split into entering and exiting trips, one can see that the The Round-Trip Approach entering trip actually has 4 excess offs and the exiting trip 2 The simplest approach is treating the round trip as a single excess ons. Corrections are made in Table 9(b) to the entering trip. However, the way an agency defines its trips is part of a and exiting trip separately, following the balancing procedure business model that carries into its scheduling database, with described later in Section 8.3. which the APC database must be consistent. Therefore, this Applying the overlapping route model requires data struc- solution is only available to the extent that the schedule data- tures that recognize loop start and end points and the rela- base is constructed in terms of round trips. tionship between the trips entering and exiting the loop. The model can be used only as part of processing the raw counts; corrected counts are returned to a database without overlap- The Terminal-Stop Approach ping routes. If the model is used in such a manner, a stop in A second way--perhaps the most common--to deal with the loop is designated as the terminal where stop records are a loop is to designate a terminal stop somewhere in the loop; inserted that give the number of bequeathed and inherited there may be a short layover scheduled there. Through load at passengers. It is easy to show that

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57 a. Attributing Boardings b. Attributing Alightings B B A' A A' A Trip Entering Loop Trip Entering Trip Leaving Loop Trip Leaving Figure 19. Overlapping loop model. Inherited Passengers = EarlyOns + LateOffs (2) both ends. Simply letting ons and offs accumulate all day long, and taking the difference as load, invites drift errors if where EarlyOns equal ons in the loop before the terminal, and ons are overcounted early in the day, even after total ons and LateOffs equal offs in the loop after the terminal. As shown in offs over the day have been balanced. Table 9(c), inherited passengers = (5+5) + 9 = 19. The only solution that has been suggested involves opera- It is also possible to have an overlapping route data struc- tor intervention: having the operator count and enter, via the ture in the final database of corrected counts. Such a data control head, the load at a certain point in each cycle, prefer- structure would allow analyses that treat the entire loop ably a point where the load is normally low. While requiring as part of both the trip entering and the trip exiting the operator input violates the traditional design philosophy of loop, with passenger movements on the loop appropriately APCs, it may be the only way of ensuring reliable load data on attributed. That data structure requires methods to prop- routes without zero-load points. Buyers in this situation can erly deal with overlapped sections. For example, the actual request that a new APC system support an operator-initiated vehicle load within the loop is equal to the sum of load on "observed load event," including an automatic prompt at the the entering trip and load on the exiting trip. As another designated location. example, care must be taken not to double count operat- ing statistics on the loop such as vehicle-miles or schedule deviations. 8.2.5 Accounting for Operator Movements Most operator off/on movements occur at layover points 8.2.4 Routes Without a Fixed-Load Point when the bus is empty. Absent counting errors, an operator or Short Loop who gets off and back on an empty bus at a layover point can be detected because such movements cause an apparent The few routes that do not have a zero-load point or short through load (arriving load minus offs) of -1, or a still more loop at either end pose a challenge for preventing drift. Exam- negative number if the operator gets off and on several times, ples are downtown circulators and routes with long loops at for example, to adjust a mirror.