This file photo shows a red light traffic camera sign...

This file photo shows a red light traffic camera sign at the intersection of Merrick Rd and Millburn Ave in Baldwin on the afternoon of May 7, 2014. Credit: Newsday / Thomas A. Ferrara

With Gov. Andrew M. Cuomo signing legislation Wednesday to allow cameras to ticket speeders in Nassau and Suffolk counties, it's a good time to look at what traffic-control cameras seem to achieve. Based on several years of data from red-light cameras, we know two things: They do seem to reduce accidents, particularly serious ones caused by drivers blowing through red lights, and the ticket revenue from them does decline as drivers change their habits.

So cameras are good for safety, but less reliable in generating dependable revenue. That's in line with what other communities have found and what experts say. The red-light cameras and the speed cameras were sold as ways to make roads safer. The bad news is that Nassau is counting on as much as $30 million per year in revenue from speed cameras, one per district to be placed near schools, to pay the costs of lifting the wage freeze for its county workers.

Nassau's revenue from red-light cameras declined about 12 percent in 2012, while accidents declined 18 percent. Rear-end collisions were down 19 percent. In Suffolk, revenue dropped 24 percent in 2012 and accidents involving injuries declined 10 percent, although rear-end collisions in Suffolk increased 20 percent.

The difference in rear-end collision results may be due to the fact that because Nassau's roads are more congested, its drivers are going more slowly and can more easily stop when cars ahead of them brake suddenly. But experts also say that Nassau instituted its program before Suffolk, so rear-end collisions in Suffolk might decline over time as drivers adjust.

Hopefully, that's true. Regardless, life-threatening injuries have decreased in both counties. Hopefully, the speed zone cameras will persuade people to drive more slowly. But county officials need to understand that this safer driving will lead to fewer tickets, and less revenue.