Speed limits are a joke and many are established to do just what the OP is talking about, generating revenues. I just drove Hw 83 from McAllen, Tx to Roma, Tx. This is the most patrolled area in south Texas and the people living along this corridor are fed up with it. I'm stopped on every trip including this one for no reason other than to hassle people, look for drugs, give tickets and generate revenues. Locals aren't happy in any of the towns along this route claiming it has hurt business and driven away tourists and rvers heading west. BTW, it is one of the most corrup areas in the U.S. with many of the sheriffs, police, mayors, and other public officials being thrown in the slammer for corruption.
Corruption On The Border: Dismantling Misconduct In The Rio Grande Valley