Evacuation route signs have popped up around Greater Bangor throughout the year.

Since April, drivers have noticed signs for emergency routes pop up all over the Bangor area and beyond. They’re big green signs with a letter — or a double letter — and an arrow under the words “Emergency Route.”

But it’s not clear from the signs when drivers should follow them, where the emergency routes start, where the signs lead and why they’re there.

Drivers don’t have to worry about learning a new set of routes, though. The bottom line is this: If drivers need to follow the emergency route signs, there will be signs on the road telling them to do so.

The emergency signs went up from Pittsfield to Medway in April, and the Maine Department of Transportation more recently installed the same signs in the Augusta area. They start on local roads within a few feet of any interstate exit. Signs with a single letter lead north and those with double letters lead south. Their placement is sporadic, with more signs on main roads such as Stillwater Avenue and Broadway in Bangor, and fewer signs located farther apart on back roads.

The signs are meant to function as permanent detours in case part of the interstate is blocked, said Steve Landry, the Maine Department of Transportation’s state traffic engineer. If a section of the highway has to be closed for a few hours at a time — for a serious traffic accident, for instance — it takes almost that length of time for crews to put up temporary detour signs, he said, which diverts resources away from dealing with traffic and cleanup on the interstate.

“If there’s a crash on the interstate we don’t want people sitting in traffic,” Landry said. “They’re in place because we don’t have the staff or the time to put [temporary detour] signs up.”

With the recent addition of signs in the Augusta area, the emergency route signs now stretch from Richmond north to Medway. Eventually there will be signs across the entire length of Maine’s interstate highways, Landry said. The department is currently working on expanding the detours south of Richmond.

The emergency routes in both directions lead from one exit to the next one along the interstate.

When it needs to activate an emergency route, the Department of Transportation will use bright pink signs on the interstate to lead drivers to the correct detour route and eventually back to the highway. Electronic signs located along interstate highways that usually warn drivers about traffic and weather conditions will also be changed to tell drivers which emergency detour to use.

The signs have no indication of the start and end points of each lettered emergency route. In some cases, the Department of Transportation’s emergency route maps show that a flagger will have to be present to direct traffic toward the interstate ramp after the signs end to avoid confusion.

The Department of Transportation has used the emergency routes only a handful of times since the Bangor-area signs went up in April.

“The first time we actually used them was the day that [Maine State Police] Detective Ben Campbell died on the road,” said Paul Merrill, a Department of Transportation spokesman. “That was the first time we actually activated the signs to alert people to follow the detour routes.”