A series of field studies was conducted at highway work zones where low-output radar transmissions were emitted (by motion detection devices) without the presence of visible law enforcement. Data were collected on vehicle speeds upstream and within the work zones, on speed changes made by vehicles as they approached the work zones, and on vehicle conflicts occurring in the 1,500-ft. approach to the work zones. The results indicated that radar signals had only a small effect on average speeds within the work zone and on the change in speeds by motorists as they approached the work zone. However, the radar signals did appear to have a slightly greater effect on vehicles approaching the work zone at speeds greater than 65 mph and on trucks. Such results appear plausible, given the likelihood of greater radar detector use among these types of drivers. The vehicle conflict study performed on the approach to the work zones found that severe braking-vehicle conflicts may increase in the presence of radar signals. There was an indication that increases in vehicle conflicts at a given work zone may depend on the amount that the average speed in the work zone (without radar) exceeds the posted work zone speed limit.