Elbow Room on the Shoulder
July 1, 2010 By: Craig Shankwitz GPS WorldDGPS-Based Lane-Keeping Enlists Laser Scanners for Safety and Efficiency
A virtual reference station network covering a metropolitan area supplies position corrections to commuter buses equipped with a driver-assist system to enable safe operation, even under harsh weather conditions, along high-volume roadways.
Bus-only shoulders on major traffic arteries enable a bus to travel on typically unused road right-of-way, bypassing congestion during peak rush hours. As the shoulder is typically only centimeters wider than the bus itself, lane-keeping becomes a key factor, and is accomplished in a pilot Minnesota project using dual-frequency, carrier-phase differential GPS (DGPS) as its primary positioning technology. DGPS provides position estimates accurate to 5–8 centimeters at a rate of 10 Hz, and is used to determine vehicle position and heading. An on-board map database is used to determine the position, orientation, and trajectory of the vehicle relative to the roadway.
Use of the shoulder as a busway offers several construction and operational advantages:
- Ease of Implementation. The shoulder exists; there is no need to acquire and develop additional right of way.
- Low Costs. The cost to strengthen and modify an existing road shoulder is significantly less than constructing a new busway.
- Routing. Because bus-only shoulders follow existing routes, no changes to bus routes, bus stops, or transit stations are needed to support bus-only shoulder operations.
- Customer Satisfaction. Transit customers who travel on buses that use a bus-only shoulder perceive a travel-time saving two to three times greater than actually realized. Keeping the bus moving at all times offers a significant psychological advantage.
- Increased Ridership. A 1997 study of bus-only shoulders in the Twin Cities analyzed more than nine bus-only shoulder routes for two years and found a 9.2-percent increase in ridership along these routes. At the same time, total ridership had decreased by 6.5 percent.
However, the use of bus-only shoulders imposes additional stress and strain on a driver. The narrow bus-only shoulder leaves a driver very little margin of error. Operating within this small margin is difficult even during the best traffic and weather conditions, and degrades to nearly impossible during heavy traffic and poor weather conditions, which are frequent during Minnesota’s notoriously hard winters.
During difficult weather and traffic conditions, the use of the bus-only shoulder offers its greatest transit advantage. If a driver is unable to utilize the bus-only shoulder, this advantage is lost. A properly designed and executed driver-assist system (DAS) enables a driver to use the shoulder under all conditions, thereby increasing schedule adherence and, as a result, rider satisfaction.
Under the U.S. Department of Transportation’s Urban Partnership Agreement, the University of Minnesota’s Intelligent Vehicles Lab (IV Lab) and HumanFIRST program, the Minnesota Valley Transit Authority (MVTA), and Schmitty and Sons Transportation will soon deploy DAS on 10 Gillig low-floor transit buses. These buses will provide express service between Apple Valley and downtown Minneapolis, a 22-mile, one-way trip.
Driver-Assist History
The IV Lab has developed and deployed DGPS-based DAS since 1995. The first deployment on public roads occurred in 2001, as part of the DOT’s Intelligent Vehicle Initiative Generation Zero Field Operational Test. The DGPS-based lane-keeping assistance was integrated with forward-looking radar for collision avoidance, enabling safe vehicle operation in zero-visibility conditions.
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