DAR ES SALAAM, Sept 6 - At least 36 people were killed and dozens injured when two passenger buses collided along a busy road in north-western Tanzania, police said on Saturday. Officers said it was still not clear what caused the accident on Friday afternoon in Butiama district, about 1,400 km (840 miles) northwest of the commercial capital, Dar es Salaam. Traffic accidents are common in east Africa's second-biggest economy, where buses are the main form of public transport between towns and roads are often poor. A total of 1,878 people were killed in road accidents in Tanzania last year, according to police data.