Local Controller described a momentary unauthorized runway entry event by an airport vehicle; the reporter noting he elected to not cancel a landing and takeoff clearance because the vehicle cleared in sufficient time.

Narrative

I was working Local Control. Runway 5/23 was closed and several airport vehicles were working on that runway. Runway 13 was the advertised runway. I had first cleared an air carrier for landing; and shortly thereafter cleared a CRJ for take off. Ground Control had issued a Sweeper Runway 13 hold short instructions at the intersection of the runways. Ground Control then observed the Sweeper crossing the hold short line and immediately instructed the vehicle to 'stop' and then 'back up'. At this moment; the CRJ had just entered the runway and was beginning his take off roll. The landing air carrier was on short final. I observed that 1/3 of the vehicle had crossed the hold short line before it stopped and started to back up. The Sweeper never was on the runway. The Sweeper was back behind the hold short line before I could have issued landing and take off cancellations to both aircraft. I issued no instructions to either aircraft; as in my judgment that was the safest course of action. I have no recommendations. Both controllers involved appear to have handled the event as effectively as possible.