The northbound A41 has been closed between Hemel Hempstead and Kings Langley while emergency services deal with a multi-car smash.
Police were called there just before 7am on Tuesday and were still there after 9am.
East of England Ambulance spokesman Gary Sanderson said: “One of the cars spun out of the car and hit the crash barrier, but our ambulance wasn’t required.”
Police said at least three vehicles were involved in the collision and they have not yet been recovered.
There was also a head-on collision between a Transit van and a silver Peugeot 308 in Flauden Lane, Felden, at 8.45am on Tuesday.
Police and paramedics were still there more than an hour later.
Police were also called to a four-car shunt between a red Renault Clio, a black Volkswagen Golf,a blue Kia Sorento and a blue Volkswagen Polo in Bedmond Road, Hemel Hempstead, at 8.40am that day. The road was not closed.