The city of Salem, Massachusetts is moving to install speed cameras in school zones. These cameras take a picture of your car when you drive above the speed limit or do not stop at a red light and can result in a ticket mailed to your home. Now Beacon Hill is considering a bill that would make such cameras legal statewide. WGBH Morning Edition Host Joe Mathieu spoke with Northeastern University law professor and WGBH News legal analyst Daniel Medwed about the legal implications of speed cameras in the state. The transcript below has been edited for clarity.

Joe Mathieu: First of all, what is the legal status of these cameras in Massachusetts? A lot of people have never heard of them. Can cities and towns just decide on their own to set them up?

Daniel Medwed: Well, here's my sense of how it works. The use of red light cameras, as you indicated, is legal in large swaths of the country — in more than 20 states. Basically, this technology captures your license plate as you commit an infraction, and that results in a notice of the violation being sent to the registered car owner's address. In some states, it's explicitly banned, including Texas this past year. Massachusetts occupies a middle of the road position. There's currently a bill pending in the Senate that would make this legal, but at the moment the way it works is that cities and towns must request permission from Beacon Hill to allow them to install this technology. That's precisely what Salem is proposing to do.

Mathieu: That's right. They passed it locally, now they're waiting for the State House to sign off on this. Any kind of technology like this, I suspect, comes with pros and cons.

Medwed: Well, the major pros — obviously the primary one — [is] public safety. The idea is that you'll deter people from speeding and thereby reduce accidents. A secondary goal is revenue generation. But I should note that under Massachusetts state law, the fine for a school zone violation is fixed at $25 per incident, which means Salem's not going to make a lot of money. In fact, city officials have indicated that they'll probably have to operate this program at a loss. In terms of cons, the big one is privacy. Yet another example of "big brother" watching over us and the fear that there might be a use of this recording for collateral purposes beyond mere traffic enforcement.

Mathieu: It can be a bit startling when people open the envelope and not only see the picture of their license plate, but their own face taken from the front of the vehicle through the windshield, depending on the system that we're dealing with. What lessons have we learned from other states?

Medwed: Well, on the one hand, the data is fairly clear that these cameras do reduce speeding. There are statistics from Virginia pointing to a 67 percent drop in red light infractions. On the other hand, scholars are somewhat mixed on whether these cameras actually reduce accidents. Some studies suggest that there is a drop through the use of them, especially if they're visible and well publicized. Other studies suggest that, paradoxically, they increase rear-end collisions because now people are braking suddenly at yellow lights, stop signs [and] school zones, and cars behind them are plowing into them. One study I saw in Chicago, I think, really illustrates the trade-offs. It detected a decrease in right-angled collisions — T-bone collisions, where a car's hitting you at a perpendicular angle — but a rise in rear-end collisions. That study concluded that these cameras are basically a wash. So I think that there are some benefits and there are some detriments. It's a little bit mixed.

Mathieu: But it's causing more people to jam on the brakes when they see these cameras.

Medwed: Some studies suggest that.

Mathieu: So, Daniel, one of the questions that we had when we first heard about this proposal is these cameras will be in effect all day [and] all night, whether school is in session or not. Is that legal?

Medwed: It is legal, and I think it's also socially acceptable if you buy the proposition that this is all about public safety. We're obviously concerned about the school day because kids might be in danger of parents dropping off kids and so on, but we're also concerned about pedestrians at off-hours. I think the biggest question is, should cities and towns have the autonomy [and] flexibility to decide on their own whether to do this? Or should we seek a statewide solution? And for that reason, I'm watching Senate Bill 2045 rather closely because it may be a way to find a statewide solution.

Mathieu: And not create yet another legal patchwork of something controversial.

Medwed: That's right.