A Local Controller witnessed an unauthorized runway crossing. The reporter suggested the use of precise and direct phraseology for ground vehicles.

Narrative

I was working Local with a DH8 on approximately a 2.5 mile final cleared to land; when I noticed a vehicle; on Ground Control frequency; cross my active runway southbound on Taxiway A without coordination. After telling Ground the vehicle just crossed [the] runway; we started discussing what happened. I did not hear the transmissions leading up to the event; only saw the vehicle during my scan. To keep this from occurring again; don't use the phrase; 'proceed as requested.' Instead state the exact taxiways and hold short or cross instructions even if they are the same as requested. Vehicles operating on the movement areas should be trained to never accept; 'proceed as requested;' as a runway crossing clearance. They should be trained to always hear the phrase; 'cross Runway XX at Y;' or otherwise question ATC to ensure you are both on the same page. Vehicles that do not need to check taxiway conditions; lights; etc. should try to use perimeter roads as much as practical; especially when a runway crossing is involved if taxiways are used. Ground/Local must always make eye contact with the vehicle/aircraft or verbally confirm the position and request before issuing instructions.