Arsonists set fire to a car on the A14 in the early hours of Saturday.
Firefighters from Huntingdon and St Neots were called to a stretch of the road between Woolley and Ellington at about 4.45am.
A spokesman for Cambridgeshire Fire and Rescue Service said: “Firefighters arrived to find a car on fire. Wearing breathing apparatus they extinguished the fire using hose reel and a dry powder extinguisher and returned to their stations by 6.25am.”
An investigation revealed that the cause of the fire was deliberate.
Anyone with information should contact the police on 101 or the independent charity Crimestoppers anonymously on 0800 555111.