Work-zone flaggers are transportation’s most vulnerable workers: 41 out of every 100,000 are killed annually in the U.S., with vehicle strikes causing more than half of those deaths. While Minnesota laws allow flaggers to report reckless drivers, post-incident reporting doesn’t prevent an immediate tragedy. To address this stark reality, UMN mechanical engineering professor and CTS scholar Rajesh Rajamani built an active defense system that intercepts driver attention before a vehicle reaches a work zone.
“Post-incident reporting is a legal deterrent, but it isn’t a physical shield,” says Rajamani. “We wanted to engineer a low-cost, automated system that acts as an extra set of eyes, demanding a speeding driver’s attention at the moment it matters most.”
In the first phase of this MnDOT-sponsored work, a research team with the UMN’s Human Factors Safety Laboratory tested initial concepts in the lab’s driving simulator, including a remote signal designed to move flaggers out of harm’s way and an electronic Stop/Slow sign mounted on a heavy wheeled base. Crucially, the team gathered feedback directly from front-line workers and work-zone supervisors, who noted that heavy equipment often goes unused.
The researchers used those insights to continue the system’s iterative design journey in the project’s second phase, aiming to balance the technology with field logistics. The result was a breakthrough design: a pair of identical, ultra-portable, 15-pound toolbox-style units with rapid-deploy stabilizing brackets. The new design completely separates the technology from the flagger’s physical Stop/Slow paddle, preserving crew mobility.
“We learned quickly that if a safety device is too cumbersome to transport, it stays in the truck bed,” Rajamani says. “Pivoting to a 15-pound standalone toolbox layout solved the human factors challenge without sacrificing the lifesaving power of the system.”
The two units act as a tag-team safety net. An upstream device, placed furthest from the workers, acts as a lookout; if its radar catches a vehicle traveling too fast, it blasts a car-horn sound and flashes bright strobe lights at the driver. A downstream unit closer to the work zone monitors the driver’s reaction. If the driver slows down after the first alarm, the system stays quiet — but if the driver ignores the warning, the downstream unit sounds a louder reinforcing alarm.
To validate the new device, the team conducted six field tests across diverse environments, including rural and urban work zones, a staged pedestrian crossing, and a tricky “speed-change zone” where open roads transition into local neighborhoods. The field test was split into two stages: first, the devices logged baseline data without alarms to ensure the radar wouldn’t trigger false alarms. Next, the audio-visual alerts were activated to measure real driver responses.
The results demonstrate the system’s life-saving potential. During a 1.5-hour test that tracked 66 vehicles, the system triggered 11 upstream alerts. Six of those drivers slowed down after the first warning, and five triggered the second alert. Of those five drivers, all but one decelerated safely. Ultimately, the dual-device setup gave erratic drivers double the physical stopping distance needed to protect the crew ahead.
Wayne Sandberg, Washington County Public Works director and engineer, served as the project’s technical liaison. “The field-testing results are encouraging for the potential future use of a low-cost, easy-to-transport device that can make work zones safer for state and county workers,” he says.
Before rolling out these smart warning systems to road crews across the state, the researchers plan to fine-tune three areas: enabling the alarms to automatically adjust as the gap between the sign and the workers changes; making strobe patterns more visible; and creating practical guidance for deploying the devices, including mounting height and spacing.
With these finishing touches, vulnerable transportation workers in Minnesota (and potentially beyond) could soon have a much-needed extra line of defense as they work to maintain the vital roadways we all rely on.
—Megan Tsai, contributing writer
