A car caught fire on the Seletar Expressway (SLE) at about 1pm yesterday after an accident with a lorry.
The Singapore Civil Defence Force (SCDF) said it dispatched two fire engines, a fire bike and an ambulance.
SCDF officers extinguished the fire with a compressed air foam jet.
Two men in their early 20s were taken to Khoo Teck Puat Hospital in stable condition.
The cause of the fire is under investigation.
Eyewitness Malyssa Malsa, a 24-year-old student, told The Sunday Times that she was in a taxi on the SLE near the Lentor exit when she saw smoke.
"There was quite a congestion from people stopping to take pictures. The heat was really bad."
She added that she did not see anyone in the car and that a lorry driver had stopped in the middle lane to take photos of the burning car.
Ms Malsa said that some motorists seemed to be calling for help.
Another person who was at the scene said the fire had been put out by the time she was there and there was a lot of foam on the road.
She said the traffic jam was "very serious", with two lanes closed.
The Land Transport Authority said on Twitter at 1.02pm that there was an accident on SLE towards the Bukit Timah Expressway before the Lentor Avenue exit, and advised motorists to avoid lanes one and two.