Police and firefighters battled a fire at a building in Garston.
Herts Police warned residents about the incident in a post on Facebook and confirmed the building that was on fire is in St Albans Road.
It is believed the site affected is the BMW car showroom.
The force wrote on Facebook: “We’re helping firefighters while they deal with a fire at a building in St Albans Road, Watford near to the Dome Roundabout.
“There are road closures in place so please avoid the area while the fire is dealt with.”
St Albans Road reopened around 4pm, when the emergency services brought the fire under control.
The closure of St Albans Road caused heavy traffic in the area and there were delays on Leggatts Wood Avenue and Colne Way.