Obaidullah Ramin, a lawmaker from northern Baghlan province, says he does not believe he was intentionally targeted by the explosion early today. Police confirmed his account.
He said the main highway leading north out of Kabul is often mined by Taliban insurgents to target official-looking convoys.
Three of Ramin's relatives were among the dead.
Roadside bombs are a favorite Taliban weapon to target government officials as well as the both Afghan and international military forces.
The insurgents have stepped up attacks in recent weeks, seeking to weaken the government and security forces as foreign troops pull back in preparation for the international coalition leaving next year.