I now have statistical data which supports the effectiveness of such a device from LCC.
Lancashire County Council Actual Results of a Speed Indicator Device
The graph shown above indicates a site in Lancashire where a SPID was put into use over a 192 hour period. The percentage of roadusers exceeding the speed limit before the SPID was deployed can be seen to the left of the graph and similarly those exceeding the speed limit after the SPID was removed can be seen at the right of the graph. The centre of the graph clearly show a significant reduction in those exceeding the limit during deployment of the SPID.
Lancashire County Council's Conclusions:
1. THERE IS AN IMMEDIATE AND NOTICEABLE EFFECT ON DRIVERS WHEN THE SPEED INDICATOR EQUIPMENT BECOMES OPERATIONAL.
2. THIS EFFECT IS MOST NOTICEABLE ON THE PERCENTAGE OF DRIVERS EXCEEDING THE SPEED LIMIT.
3. THERE IS A SLIGHT RESIDUAL EFFECT AFTER THE EQUIPMENT IS REMOVED.