Which device is essential for measuring vehicle speed compliance?

The device that is essential for measuring vehicle speed compliance is the radar speed gun. This tool is specifically designed to accurately determine the speed of a moving vehicle and is frequently used by law enforcement officers for monitoring traffic speed. By sending out a radar signal that reflects off moving vehicles, this device provides real-time speed readings which can be compared to posted speed limits to identify violations.

While other devices listed may have their own applications, they do not primarily focus on speed compliance monitoring. For instance, a speedometer is built into vehicles to show the driver their current speed but does not measure compliance against speed limits for external enforcement purposes. Traffic light cameras are used mainly for monitoring red light violations instead of speed compliance. A GPS navigation system can provide speed information but lacks the enforcement capabilities and real-time speed monitoring essential to measuring speed compliance accurately. Hence, the radar speed gun is the most effective tool for this specific purpose.