I would suggest using Round up with the addition of Imizapic. Normally you can find this combo in Lowes. Round up acts through the leaves of the weeds, and the imizapic works through the roots and is residual in the ground. Just follow the instructions and it is quite safe.
As another mentioned there is a Round-up product now that is residual. They claim it lasts something like 8 months. I used some (bought the concentrate) last summer on my very weedy driveway and it worked pretty well, took the weeds about a week to die and prevention did last for 3-4 months.