I once almost got T-boned by a cop that was doing somewhere closer to 60mph in a 40 with no lights in a Caprice. His speed was later corroborated by several of the people who were standing in the lot I was leaving(which was a snowball stand with a long line, so no one has anything better to do than to stare at traffic while they wait). This was during heavy traffic, and on a three lane road with lots of side streets(I was making a left across two lanes). The officer was traveling in the center lane and was just in front of a shop I usually use as a reference point when leaving this lot. On a highway is one thing, but without lights it's hard to tell that a cop is doing more than 1.5x the speed limit when you're sitting still, especially when they're masked by heavy traffic. He closed the gap between me and that shop in 3/4 of the time it normally takes. If I hand't seen him at the last second and floored it, I'd probably be dead, I'm only lucky that I pay attention for the occasional warp-speed idiot on that road anyway, especially since I'm small car in a pickup-dominated part of the country. I was just kind of surprised that it was a cop driving like an idiot that almost got me.
There's never a good enough reason to put innocent lives in that position, if it were an inexperienced driver in a slower car the cop would have killed them. I can understand not having audible sirens at all times, but flashing lights go a really long way toward making it known that you're going to be moving at speeds that normal traffic wouldn't.