These mobile devices display the drivers travel speed on a large display along with the posted speed limit. Their goal is to encourage compliance.
Their effectiveness tends to be limited to the times when they are displayed - with minimal effect on speed when they are removed. A drop in speed of 25 percent is possible with the devices when displayed.
After a series of applications an overall drop in speed of about 6 percent has been found when traffic volumes are less than 600 vehicles per day. On higher volume streets their impact has been found to be negligible.