The convoy was made up of hundreds of people who fled their homes in Fort McMurray for oil worker camps north of the city.
The fire is expected to double in size in the coming 24 hours.
The fire now covers an area larger than New York City, and more than 80,000 people have fled their homes.
Tens of thousands of people have been evacuated by air with 300 flights to the provincial capital Edmonton since Tuesday, and another 4,000 are due to leave on Saturday.
The police-escorted road convoy of 1,500 vehicles was due to pass by the southern part of the city but was suspended on Friday afternoon until Saturday.
Twenty minutes south of Fort McMurray, the road forks into two branches. By noon on Friday, both were ablaze on either side.
We watched with the police as the skies filled with grey and black smoke and flames roared into the air, devouring even the tallest pine trees.
The danger, said one officer, was “tentacles growing out of the fire”, which could end up looping around and trapping people.
There are no reports of injuries or deaths but several residents who may have survived the fire have apparently been spotted in Fort McMurray. Teams are now sweeping the city in case any homeless people were left behind.
There is also concern about oil facilities, particularly near Nexen’s Long Lake oil extraction site.
“We’re looking at a blast area of about 14 kilometres if that plant were to go,” said Sgt Jack Poitras.
“We stopped due to heavy smoke,” said Sgt Jack Poitras of the Royal Canadian Mounted Police. “You get flames of 100, 200ft up in the air on both sides of the road so it’s not safe.”
The provincial government in Alberta has declared a state of emergency and will provide C$100m ($77m) in cash to evacuees.
Alberta Premier Rachel Notley said the damage was extensive and would take months to repair.
She said: “The downtown is largely intact. The hospital is still standing. The telephone centre is intact. The water treatment centre is back up and running. Municipal buildings and the aeroplane remain intact.”
The city is in the heart of Canada’s oil sands country, and the region has the world’s third-largest reserves of oil.
As much as a quarter of the country’s oil production has been halted by the fire, raising concerns about the effect on the Canadian economy.
Two oil sand sites are directly threatened by the blaze while 10 operators have cut production.
In a statement released on Friday afternoon, Prime Minister Justin Trudeau called upon Canadians to donate to charities assisting relief efforts.
“I would once again like to thank the many first responders who are working tirelessly, day and night, to fight this fire,” Mr Trudeau said.
“To those who have lost so much: we are resilient, we are Canadians, and we will make it through this difficult time, together.”
More than 1,000 fire fighters and 150 helicopters, 295 pieces of heavy equipment and 27 aircraft tankers have been deployed, according to the Canadian government.
But Chad Morrison, Alberta’s manager of wildfire prevention, said that what they really need is rain.
“We have not seen rain in this area for the last two months of significance,” Mr Morrison said.
Without rainfall, officials predict that it could be “weeks and weeks” before the fire is completely out.