It was revealed after Conall McDevitt, SDLP MLA for South Belfast enquired about peak time traffic in Greater Belfast in the assembly.
The figures come from average speeds recorded in the Roads Service Journey Time Survey from 2011 on 11 predetermined arterial routes were detailed by Regional Development Minister Danny Kennedy.
During 7.30 to 9.15am the slowest average speed of 6.37mph was recorded inbound on the Ormeau Road.
This was followed by average speeds of just over 10mph inbound on the Newtownards Road and Falls Road.
Responding to the findings, Mr McDevitt said that Mr Kennedy must do more to tackle congestion in Belfast.
"The figures I relieved from the Minister in response to my question show that there is much to be done to reduce congestion across the city at peak times," he said.
"Regular commuters will not be surprised to see that many journey speeds on arterial routes in and out of the city do not pass 10mph at peak times, but many have no other option, particularly those from rural areas, who are poorly served by public transport.
He continued: "The Department must promote access to the city, both for those with access to public transport and those without, as traffic speeds of 6.37mph are unacceptable.
"We need a lot of investment in better park-and-ride facilities, more frequent buses from those facilities, we need to really make it attractive for people coming into town to want to use public transport."
The MLA said he will be introducing a Private Members Bill to reduce the speed limit on urban residential roads to 20mph to encourage safer and consistent speeds.