A bright line doesn't only make it clear what's illegal - it also makes it clear what's
legal.With a less clear law, how would you know if your speed was legal?
I think I'm a reasonable person and while sober, well-rested and attentive on an empty freeway at sunset on a road drying after 0.1 inch of rainfall in the preceding hour with no other cars in view driving eastbound at 70 mph would be reasonable.
But what if a cop is having a bad day, or has a quota to hit, or doesn't like my bumper sticker, and decides the legal limit is 65 mph because the setting sun might be dazzling in my rear view mirror?
And if I don't want the cost and inconvenience of going to court, should I go at 60 mph instead, just in case?
The reality is the courts are very slow and very expensive for decisions that are being made millions of times per day.