Najibullah Samsur, a local police chief, said the bomber wounded two Afghan civilians in the attack.
Witness account
The blast destroyed one vehicle, which was flung to the side of the road, one witness said.
"There were two foreigner vehicles that passed near my shop and a third private vehicle hit them, causing the explosion," Ahmad Shah said.
Shah, whose shop is close to the blast site, said the attacker's vehicle and two of the contractors vehicles caught fire after the explosion.
The foreigners from one of the lightly damaged vehicles fired their guns into the air to prevent civilians from approaching, Shah said.
The Taliban, who have been fighting foreign troops and the Afghan government, have carried out a series of attacks this year.
A suspected Taliban suicide bomber killed more than 20 police in the heart of the city on June 17.
Mine clearers released
The Taliban released 18 Afghan mine clearers captured six days ago, but have held on to their equipment and three sniffer dogs, the movement and an official said Thursday.
Nine of the men were released late on Wednesday and the remaining nine early on Thursday, Shohab Hakimi Mohammad, head of the Mine Detection and Dogs Centre, said.
"They are all freed. We are very happy. They were a bit depressed but OK," he said.
The miners were captured early on Saturday in the southern province of Ghazni's Andar district, about 120km south of Kabul, as they were driving to a minefield that they were working to clear.
The Taliban's "leading council" had decided the men should be freed because they had been working in Afghanistan for years, Yousuf Ahmadi a Taliban spokesman, said.
"They were advised and they were set free," he said. The Taliban has warned Afghans against working for foreign companies and military forces.