I can't really make sense of what you're saying. If the road is straight and wide but also has a low speed limit, that's not sending mixed signals. Rather, it's suggesting that you should drive slow even though your instinct tells you that you could drive quickly, presumably because there are either obstacles creating blind points that could lead to pedestrian or bicycle involved accidents, small children playing nearby, or cars turning onto or from side roads that you might strike if you're driving at the speed that your gut tells you is safe.

In other words, you shouldn't trust your gut when deciding how fast is safe on a road because your gut is often mistaken about the finer points of road design.

Also, you wrote that a slow driver would be an obstruction to other vehicles including trucks. I think you were wording that as a bad thing, but in reality it's a good thing. One reasonable driver can force a dozen bad drivers to slow down.