What an intrusion alarm does
When a vehicle leaves the traffic lane and heads into a work zone, the crew has seconds. An intrusion alarm makes sure they hear or feel the warning in those seconds. There are two kinds. Boundary alarms trigger when something on the edge of the work zone is struck or crossed, such as a sensor cone in the taper. Predictive alarms track approaching vehicles with radar and sound when one is about to cross into the work zone or cannot stop in time.
The time budget
The crew's warning is the time a vehicle needs to travel from the point where it is detected to the work space, minus the alarm's own delay. Distance and speed do the rest. The table uses LaneWarden's design values: a car tracked from 460 ft before the taper, and the alarm sounding within one second.
Trucks are tracked from farther out, about 590 ft, so they give the crew one to two seconds more. Every second matters: at 54 mph a vehicle covers almost 80 ft in one.
Could the driver still stop?
The second question is whether a driver who reacts at the taper can stop before the work space. At a flagger station there are 670 ft between the taper and the crew. At 54 mph a driver needs about 480 ft to react and brake, so stopping is possible. Above about 66 mph it is not, and the alarm becomes the crew's only margin. A lane closure's longer merging taper leaves 1,290 ft, enough at any legal speed.
Two layers, independent of each other
Each kind of alarm has a blind spot. Radar can be blocked by other vehicles or surprised by an unusual path, and a boundary sensor only triggers once something has already been hit. Used together, each sounding its own alarm, they cover each other. Neither should depend on a cellular network or a server: the decision to alarm has to be made on site.
Crews work next to compressors, saws and traffic, and may not hear a siren. Wearable alerts that vibrate, as well as flash and sound, reach the people the alarm is for.
What an alarm cannot do
- An alarm does not stop a vehicle. Traffic control design and positive protection, such as barriers and truck-mounted attenuators, come first; federal rules (23 CFR 630, Subpart K) ask agencies to consider positive protection where workers are exposed.
- False alarms erode trust quickly, so detection must be tuned to the layout and tested on a closed course before it protects a crew.
- Detection ranges and alarm delays are design values until they are measured in the field.
Try the numbers
The worker protection page has an interactive version of this calculation. Change the speed, the vehicle and the layout, and see the warning time and stopping distance update.