The latest addition to Oregon's regional-transportation and traffic- management system for the greater Portland area is the integrated bus dispatch system (BDS), operated by the Tri-County Metropolitan Transportation District of Oregon (Tri-Met). The system links buses and command centers using sophisticated technologies that enable dispatchers to respond to changing events and conditions in near realtime, while managing 850 fixed-route and paratransit buses over 650 square miles. In addition to complementing Portland's light-rail system, the BDS has enabled Tri-Met to deliver improved bus service with greater reliability and security.

At the heart of the BDS is the ORBTRAC 300 Fleet Management System, developed by Orbital Transportation Management Systems, of Virginia. Basically a sophisticated Automatic Vehicle Location System (AVLS), it uses data from the global positioning system (GPS) and GIS spatial analysis to provide dispatchers with realtime bus locations, operational data and security communications. The main elements of the ORBTRAC are the GPS satellites, a differential correction station and onboard sensors. At the command center, nine UNIX workstations with two color monitors display bus locations and movements and a prioritized list of events requiring action by the dispatcher or driver.

A GPS receiver aboard the bus determines its position by triangulating on signals received from three or more satellites. Since the signals are affected by atmospheric conditions such as rain and snow, and by nearby buildings and trees, as well as by selective availability, a location derived from raw GPS data can be off by as much as 100 meters. To correct such errors, a computer interfaced with a GPS receiver at a stationary site continually compares its known location with the one reported by its receiver. The computer calculates the difference and broadcasts corrections, which are picked up by the AVL aboard the bus and applied to its raw GPS location data, correcting the location of the bus to within 10 meters -- about one bus length.

The onboard computer automatically relays its corrected location to the command center, along with data from onboard sensors that monitor passenger count, door motion, wheelchair lift and other bus operations. The onboard computer also provides two-way voice and digital communication between drivers and dispatchers. If the bus is off-route, behind or ahead of schedule, the computer automatically sends that information to the command center. At the end of each day, the computer uploads the day's recorded schedule and route data to the command center, where the information is automatically summarized and stored for later analysis. On newer buses, computers automatically change the bus-destination sign, and count passengers as they board and alight.

Command Center

At dispatcher workstations, bus movements are displayed as colored icons moving along different routes. The movement of fixed-schedule buses are monitored for sequencing and spacing, ensuring that buses are on- route, on-time and not bunched up. The dispatcher can send messages to an individual bus or to all buses simultaneously. In the event streets or bridges are blocked by construction or a traffic accident, digitized detour instructions can be sent simultaneously to all buses on the affected routes. The second monitor displays computer-prioritized data from the buses, enabling dispatchers to quickly see events requiring first response.

The data from the buses automatically uploaded to the command center are summarized into tables for the monthly and quarterly reports and stored in a database for three months. Tri-Met analyzes the summaries for flagged irregularities, ridership patterns and complaints, driver performance, maintenance reports and sources of data inaccuracies. The objective is to assess the effectiveness of the overall system and adjust schedules to meet constantly changing priorities for public transit.

The Role of GIS

According to Tri-Met Coordinator for Service and Performance Analysis Steve Callas, the BDS uses a ESRI basemap maintained by the Regional Metropolitan Government. Tri-Met's GIS department provides the map overlays -- routes, stops, shelters, etc. --