Speed studies are used to determine an appropriate speed for a road, or to determine if the road has a speeding problem. The US Department Of Transportation and Federal Highway Administration provide a quality webpage describing speed studies. I reference the Handbook of Simplified Practice for Traffic Studies developed by Iowa State University's Institute of Transportation for my speed studies.
There are two numbers to look for in speed studies: The 50th percentile also known as the average or mean, and the 85th percentile. A typical metric for establishing a speeding problem is a mean speed of 5 or more miles per hour (mph) over the posted speed limit (VDOT, 2008). Other agencies consider an 85th percentile speed which is 7 to 10 mph over the posted speed limit as a speeding issue.
I use both calibrated radar and lidar to collect speed measurements. Radar uses the Doppler effect which emits pulses of radio or microwave energy from an antenna. These radio waves bounce off objects and return to the radar. By measuring the precise time it takes for the signal to echo back and the change in frequency (Doppler effect), the radar determines an object’s speed.
Lidar works by emitting rapid, invisible eye-safe laser pulses—about 200 per second—that bounce off objects and return to a sensor, which calculates distance to and speed of an object based on the travel time. Lidar has a much more narrow beam than radar. At 1000' from the lidar, the laser beam is only 3 feet wide. The radar I use has a 12 degree beam which encompasses an area of just over 200 feet wide at 100'. So in high traffic density areas lidar allows me to better determine the speed of a specific vehicle.