I often see this at the bottom of speed-limit signs.

Problem:
Neither meaning of the phrase makes sense.

Explanation:
“RADAR ENFORCED” could mean either “(THE PRESENCE OF) RADAR IS ENFORCED” or “RADAR-ENFORCED” (note the hyphen).

The former makes no sense, given that drivers don’t have to have radar.

The latter makes no sense, either, given that enforcing a speed limit with radar would mean that a police officer could slow a vehicle to the speed limit by beaming radar toward it. If that were possible, it probably would kill the driver in the process.

What the sign posters want to say is that the law-enforcement authorities are monitoring vehicles’ speeds with radar (detectors).

Solution:
“MONITORED WITH RADAR”