The New Hampshire political primary election is just a month away – it’s on Sept. 10 – and the state Department of Transportation recently sent out its regular advisory on placing political signs near highways. This gives us the excuse to dust off our regular explainer about the topic:

Q. Where can campaign signs be placed?

On private land, owner’s permission is necessary. On public land the owner’s permission is also necessary, so don’t stick your sign in front of town hall unless the select board says it’s OK. Which they probably won’t.

Rules for the land alongside roads, the most coveted real estate for campaigns, are a little complicated. New Hampshire Department of Transportation guidelines can be summarized thusly:

■Signs cannot be placed on interstates or along on- and off-ramps, period.

■Signs can go next to all state roads unless they are deemed to create a hazard by impeding the view of traffic or if they get in the way of maintenance work like mowing. In that case, they can be removed by road crews or the police.

■Police can even remove signs from private property if they block the view of traffic traveling along the road.

■Signs and placards cannot be put on utility poles. In fact, you’re not supposed to attach anything to utility poles, despite what owners of lost pets think.

■Campaign signs removed by local officials are stored at the public works garage or police department. If nobody gets them, they usually get tossed a week or two after the election.

Q. I see an illegally placed sign. Can I remove it?

If it’s on your own property, go ahead. Otherwise, no.

State law is clear: The only folks who can remove a sign are (1) the campaign that placed it, (2) the property owner, or (3) law enforcement or highway crews.

Signs must be removed from non-private land by the second Friday following the primary, unless the candidate(s) won. In that case the sign can stay until the general election in November. They can stay on private property forever if the owners don’t mind.

If you see signs on public land after the deadline – Sept. 20 – then call your local police or public works department.

Q. Do roadside political signs work?

People think they do or we wouldn’t see all the news stories about candidates complaining that their signs were stolen. We know of only one effort to scientifically study their effectiveness.

A paper in the March 2016 edition of the research journal Electoral Studies discussed four randomized trials involving candidates – for Congress, mayor and a county office – in a few different states, as well as a campaign directed against a candidate. Signs were placed in some precincts and not in others for each race.

The paper’s conclusion was that, on average, signs increased a candidate’s voting share by 1.7 percentage points. That’s not much but it’s not nothing: Plenty of elections are decided by less than 1.7 percentage points.

So it seems signs do sway voters a little bit, sometimes, probably.

Talk to political operatives, however, and you’ll learn that the target audience for signs isn’t really voters – it’s donors and volunteers. People are much more likely to give their money or their time to a campaign if they think it’s doing well and local signs can enforce that idea.