In my town (a suburban town with a low crime rate), the police spend the majority of their time enforcing traffic laws. The municipal courthouse is always filled with people, and if I had to guess, I’d say that 80% of the people are there as a result of a traffic violation, while the other 20% are there for drug offenses/other.
It seems like autonomous cars would lead to a drop in traffic offenses and thus revenue. Even if only a tiny portion of the town’s revenue actually comes from traffic violations (when compared to local property tax), there would probably be a lot more idle time for police.
I suspect that they will first start to raise our property taxes to compensate for the lost revenue, but I would think that the long term effect would be a reduction of municipal workers/police.
Of course, this doesn’t apply to other areas like Philadelphia, where police spend only a small percentage of their time enforcing traffic laws.