In Minnesota and across the country, the most common traffic signals are those that are actuated by a device embedded in the road that detects the presence of a vehicle at the intersection. While these embedded devices can last for decades, they don’t always detect modern vehicles, such as electric cars, or vulnerable road users, such as bicycles or motorcycles. New research evaluated the performance of video- and radar-based detection systems, which are becoming more readily available, under a variety of Minnesota weather conditions.
For decades, traffic signals have relied on sensors embedded in the pavement to detect and respond to vehicles waiting at an intersection. These devices, or loop detectors, are the industry standard for identifying large steel objects and can provide maintenance-free service for years when installed correctly as part of a road-building project.
However, replacing or adding new devices to existing roads requires cutting into the pavement. This step makes the detectors significantly more expensive and reduces the life span of both the device and the road.
Another drawback of embedded loop detectors is their inability to identify smaller vehicles and those that aren’t made of steel, which limits service and mobility for vulnerable road users. Because Minnesota’s city and county engineers are committed to making roads safe and accessible for everyone, this project tested a variety of nonintrusive technologies (NITs) for traffic detection to compare the performance and long-term costs of several commercially available options.
What Did We Do?
The project began with an investigation into various NIT systems on the market and a review of the literature that has been published regarding their performance. Since a number of transportation agencies in Minnesota have already begun to use NITs at select intersections, interviews with traffic operations staff from Hennepin County and the City of St. Paul provided valuable insight into the agencies’ real-world experiences and the benefits, challenges and costs of operating and maintaining the different systems.
“Nonintrusive systems can have some benefits over traditional vehicle detection systems, but they also have drawbacks. This research identified what should be considered so that local agencies can make better-informed decisions when choosing which system is right for them,” said Derek Lehrke, signal operations and construction engineer, MnDOT Metro District.
Based on those discussions, the NITs most commonly used in Minnesota were selected for further evaluation. These systems, which employ high-definition cameras and radar, were tested at six intersections in the Twin Cities area under a variety of conditions.
What Did We Learn?
Traffic operations staff reported that weather and environmental conditions greatly affect NIT performance. They noted that glare, as well as snow, ice and dirt that accumulate on the camera lens, can limit a system’s visibility and reduce its effectiveness. Adding sun shields and heated lenses can mitigate these effects. Routine maintenance to clean and adjust cameras is also important but can add significant costs over the NIT’s service life.
The comparison of the NIT products in the field revealed that no system clearly outperformed others in all weather conditions, though some products seemed to have fewer issues in certain scenarios. Installation costs were lower for NITs than for traditional embedded loop detectors. Researchers estimated that placing NIT devices at a sample intersection could cost approximately 1.5 times less than the costs to install traditional loop detectors.
Using the information gathered from this project, researchers developed a new decision tool to help traffic operators select, install and maintain the right NIT products for their needs. They also produced several recommendations for improving NIT durability and performance. A sampling of these best practices is provided below:
- Angle cameras below the horizon line and install glare shields to reduce the effects of external light sources.
- Add filters to cameras to reduce glare, and re-position cameras when necessary.
- Use a central monitoring system to allow operators to perform routine checks and diagnose NIT problems remotely.
- Consider local weather conditions when selecting NIT products for a particular intersection.
- Install heat shields on all NIT devices to help with snow and ice removal and to reduce the need for on-site maintenance.
What’s Next?
While NITs offer several advantages over traditional systems, they also have limitations. Selecting an appropriate system can be challenging. Though NIT products and systems are continually evolving, this project highlighted the core features so that city and county engineers across Minnesota can choose the system that suits their local environment and community.
Crossroads 