Here's the link to the video footage of the Lahore 27 May 2009 (27/5 as the Mrikans would label it) blast:
Luqman, the expert analyzer is wrong when he says that the van was so heavily loaded with explosives that the driver couldn't move it forward!
Such a van can carry a lot of weight; just watch the Lahore public transport to see what I mean; they load twice as many commuters in there, 16 hours a day, everyday!
In the Marriott Hotel blast of Islamabad, a dump-truck was used on 20 September 2008; people there reported that the driver drove erratically!
And for this one too (27 May 2009, Lahore), the driver was certainly a novice who struggled with the gearbox and probably restarted the vehicle several times.
If the van could move swiftly on the road AND jump the traffic lights, it could also move easily and with speed later. Hence, the start-stop action is due to the novice driver. When faced with certain death, he may have become confused or scared.