CHELMSFORD -- State police said a tractor trailer truck entered Route 3 northbound from a southbound ramp at Exit 29 and collided with one other vehicle around 7:30 p.m. No one was hurt, but traffic was backed for a short while, state police said.

The ramp was temporarily closed but was reopened by 8 p.m.

The accident remained under investigation, state police said.

State police said the accident occurred when the tractor trailer driver got on the wrong way and tried to correct himself.