A VAN packed with flammable chemicals used in the process to make the high explosive TNT caught fire on the M62.
The flatback van was being driven on the westbound carriageway near junction 23 at Outlane when a piece of cardboard fell from the back on to the exhaust.
This ignited and spread to the heavy duty PVC cover that was strapped over the load.
Watch manager Peter Rushby, of Elland fire station, said: "When we arrived the cover was well alight, but we were just in time to stop it spreading to the load.
"If that had happened we could have been dealing with a very serious incident indeed."
The load contained four 50-gallon barrels of toluene.
This is derived from petrol and used as a solvent in aircraft fuels and for making resins for adhesives, pharmaceuticals and as a disinfectant.
It is also used in the process which makes the powerful high explosive TNT.
Two men in the van were unhurt in the drama.