In what seems to go against some conventional thinking on the motivation to be corrupt:
Not only did the police spend more time shaking down drivers after they received higher salaries, but they took significantly more money after receiving higher wages. Instead of leading to a more (if the reader would permit us) civil police force, the higher salaries seemed to give them a higher appetite for corruption. Although we were initially encouraged to spot a parallel increase in the number of trucks that didn’t pay bribes, we were disheartened to realize that bribery did not actually change. This was partly because the police salary change also increased the number of road stops on the highway. Arguably worse, the aggregate bribe cost of the trip significantly rose following the salary increase.
This, from a World Bank blog by Kweku Agyemang. As the author reports, this happened in spite of drivers all having valid documents.
…policemen who received the single spine salary increase in Ghana increased the effort they allocated to collecting bribes in time spent asking for bribes, in the number of checkpoints they operated, the value of bribes they took, the total amount that truckers had to pay on the road, all while they increased the number of trucks let go without a bribe
So the higher salary seems to have upped the stakes for these cops in the bribe-taking game.