What requirement was established regarding vehicles using RADAR when stationary?

The requirement that vehicles using RADAR when stationary must be visible from 500 feet is essential for ensuring safety and effectiveness in speed enforcement. This visibility rule helps ensure that drivers have adequate notice of the speed enforcement operation, which can encourage compliance with speed limits. Being visible not only increases the likelihood that drivers will reduce their speed upon seeing the enforcement but also promotes transparency in law enforcement practices.

Visibility from a significant distance contributes to both the deterrent effect of speed enforcement and the safety of the officers conducting the operation. Adequate visibility allows motorists to recognize the presence of law enforcement and adjust their behavior accordingly, which is a key aspect of promoting road safety.

The other options, such as requiring sirens, visible lights, or a reflective coating, do not directly correlate with the specific need for the RADAR-equipped vehicle's presence to be noticeable from a considerable distance as a means of effective enforcement posture.