I don't know where you live....but here in the great state of NY, speeding fines are a great revenue generator. And as such, the town, county and state PD just drive around with their radar on at all times, just hoping to catch you snoozing on a road that is perfectly safe for 90 mph, but instead is marked 40 or 50 or 55 or maybe 65 mph. Radar detectors help prevent those little lapses that we all have. RD's are also nice when I'm behind some jackhole going 50 in 55, and I need to quickly blip up to speed and get in and out of the passing lane in a few seconds, and not worry about some traffic maid popping me for say 75 in a 55 when I'm performing a perfectly safe pass.
On the highway, sure I set it for 9 mph over the limit, and cruise along, no RD needed for those trips.
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