Cold and snowy winter weather conditions create several unique challenges for automated vehicle (AV) operations. To better understand operations of an automated shuttle bus in winter weather conditions, the Minnesota Department of Transportation (MnDOT) conducted an Autonomous Bus Pilot project.
The test vehicle, with no steering wheel or brake pedal and a Society of Automotive Engineers (SAE) Level 4 autonomy classification, was able to navigate autonomously using pre-mapped routes. An on-board localization system that included GPS, LIDAR sensors and an odometry and inertial measurement unit allowed the automated shuttle bus to operate accurately on the pre-programmed route. The shuttle bus was equipped with four-wheel drive, winter tires and an interior heater.
A demonstration of the automated shuttle bus was held for select transportation professionals in December 2017 and January 2018 on a controlled test track at MnROAD in Albertville, Minnesota. This was followed by public demonstrations of the automated shuttle bus near Nicolett Mall in downtown Minneapolis, Minnesota between January 24th and January 28th in conjunction with community activities that preceded Super Bowl LII. Five additional demonstrations were held between February and April of 2018 at 3M, the City of Rochester, the University of Minnesota, Hennepin County, and Bismarck, North Dakota.
Key findings from the project are summarized below:
- The automated shuttle bus operated well under dry pavement conditions with no precipitation. The vehicle kept a safe operating distance from other vehicles, pedestrians, bicycles and other roadway obstructions on the track, performing slowdowns and stops as needed. Daytime and nighttime light conditions did not impact the shuttle performance.
- Falling snow, blowing snow, or loose snow on the track was often detected as obstructions by vehicle sensors, causing the vehicle to slow down or stop to avoid a collision. A key finding observed on a clear day with bitterly cold temperatures was the detection of exhaust fumes as an obstruction from a car traveling in the same direction in the parallel lane, causing an unplanned sensor activated slowdown and emergency stop.
- Snow banks alongside the vehicle routes caused issues with pre-programmed paths. Snow banks had to be removed at the Minnesota Capitol demonstration and the Hennepin County demonstration was delayed a week from plan to allow the snow banks to melt.
- At times, compacted snow and patches of ice or slush on the track caused the wheels to slip, which in turn created issues with the bus not responding to its exact location on the track.
- Salt spray from treated sections of roadway that collected on the vehicle sensors did not appear to significantly degrade performance. While some minor anomalies were observed, the reason could not be confirmed. Cleaning dirt accumulation from the sensors due to normal operations appeared to improve the automated shuttle bus performance.
- Because of the rural nature of the MnROAD site, the vehicle required installation of localization infrastructure. Signs posts were installed approximately every 100 feet around the test loop.
- As the core temperature of the battery dropped significantly, automated shuttle bus operations were negatively impacted. The project testing staff observed that colder winter weather temperatures had the effect of discharging the battery faster. During periods of subzero temperatures, the vehicle batteries discharged more quickly when the vehicle heater was running.