It seems they write tickets by error:
http://www.theverge.com/2014/1/24/53...urs-corp-xeroxThe Baltimore Sun obtained data from a secret audit conducted by the city that revealed that Doty's is likely one of thousands issued erroneous speeding tickets by the city's cameras. The audit, which was conducted by consultant group URS Corp, concluded that the error rate was more than 10 percent for the city's speed cameras, costing residents $2.8 million in tickets. One camera URS audited turned out more errors than correct citations.
jtk