[July 4th]

If they were driving at the legal speed limit, how were they impeding traffic?

Nobody said the law was perfect. If you're doing 55 in the left lane of the Dan Ryan expressway in Chicago, you are nowhere near the requisite 80 miles an hour, and you can/will be pulled over for impeding traffic (as well you should be). Just because you're not breaking one law doesn't mean that you're not breaking another.

[Republican Wildcat]

That doesn't make any sense. You can't be cited legally for impeding someone's "right" to go 25 MPH over the speed limit especially if you are already going the maximum allowable speed.

[Young Scholar]

So you could theoretically be pulled over for speeding if you're moving with traffic and for impeding traffic if you go the speed limit? Is there a legal way to drive?