I take a main city road every day where the flow of traffic exceeds the speed limit. It's 30 mph (I did the conversion), and traffic goes 45 mph. Go below this and people will gun it and pass you, sometimes quite narrowly. Once every twenty days or so I drive by a speed gun and I see a wave of brake lights as everybody slows back down and continues on a lot slower for about a block or two, then continue on. If you're a cop would you be more concerned with A. All cars to obey posted limits or B. The flow of traffic to move smoothly and safely, even if they're exceeding the limit. Assuming that road conditions and visibility is ideal, and people aren't screaming by a triple digit speeds.