A FALLEN tree branch or animal movement was apparently responsible for the setting off of a "slope alarm" in the roadworks zone at Cunningham's Gap which closed the highway on Tuesday night.

The closure came just hours after a truck and ute collided nearby on the highway at Mount Edwards, which forced a closure of all lanes for two hours, from 1pm to 3pm.

A Department of Transport and Main Roads (TMR) spokeswoman told the Daily News yesterday the alarm - intended to warn engineers of any rock movement on the Gap cliff face - went off about 7.30pm on Tuesday.

"We continue to conduct regular inspections and monitor the slopes above the highway at Cunningham's Gap using geotechnical equipment," the spokeswoman said.

"The highway at the Gap was briefly closed after one of the four slope alarms went off on the slope to the west of Gap Creek Bridge."

"The construction contractor and a geotechnical engineer were on site at the time and the road was immediately closed to allow the engineer to inspect the area.

"It has not been determined what triggered the alarm, with the most likely cause being a fallen branch or an animal, but the inspection determined the slope was stable."

The spokeswoman said the "short-term" road closure allowed the TMR staff to check the slope was safe for passing traffic before the highway was reopened.

Work on the multiple zones across the Gap is expected to continue until mid-2012 and cost at least $40 million.

Meanwhile, Boonah Police yesterday said Tuesday's crash at Mount Edwards was "still under investigation".

A witness who contacted the Daily News yesterday clarified that the blue Toyota Hilux utility involved was heading to Brisbane when it was overtaken on the outside lane by the truck, which was carrying a load of sand.

The witness said it appeared that the Hilux driver had for some reason turned into the path of the truck.

The driver of the Hilux was treated at Ipswich Hospital for a broken collarbone.