Spraying roundup on weeds in the cracks of your driveway is kind of silly. Boiling water will do the same job better.
Pour it straight onto the weed and down into the crack. The heat ruptures the plant on contact and it wilts within the day. A stubborn deep-rooted one might need a second pour, but that's the whole method.
Roundup's own label tells you to keep it out of storm drains. And a driveway is basically a funnel straight into one. Every rain carries whatever you sprayed down the gutter and, in most towns, straight into the nearest creek with nothing in between.
Boiling water kills the weed and stops at the weed.