A certain Canadian video comes to mind....Although the speed limit is 65, everyone consistently drives 75 to 80 mph. If your driving the speed limit on that section of the 14 freeway, you're actually a hazard. It's the only freeway I know where the CHP won't pull you over unless your going more then 20 mph over the limit. Simply because EVERYONE is driving that fast.
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If it's not unsafe to have most drivers at 10mph over the posted limit, then the posted limit is wrong.Because they're going 20 mph over the speed limit, they likely don't have the sight distance required to have even a reasonable chance of avoiding anything like a stopped car. Speed limits aren't just for revenue enhancement--although some are, and those train drivers to ignore the speed limits.
If it is unsafe to have most drivers at 10mph over the posted limit, then the lackluster enforcement is wrong.
In short, no matter what the drivers are doing the governing and/or policing bodies are incorrect in at least one way.