Air Traffic Control Developmental
Communication Breakdown
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance
Procedure
I was working Local and Ground control positions combined.  Conditions were VFR and Rwy 13 was in use.  I had approved Vehicle X onto Runway 13 for maintenance work.  A few minutes later I also approved Vehicle Y onto Runway 13 for maintenance work as well.  I had both vehicles written down onto my scratch-pad and also had the 'Firetruck' (a local memory-jogger aid our facility uses to delineate when a vehicle has been approved onto the active runway) out and displayed on my strip-bay and had activated the RIPD (Runway Incursion Prevention Device) which is an aural aid to remind the controller there is a vehicle on the runway.  Aircraft X was inbound for the Visual Rwy 13 approach from the SW.  The aircraft was still about 10 miles out on a modified right-downwind for Runway 13 when he check onto the Tower frequency.  I told Aircraft X to continue inbound; vehicle exiting the runway.  I instructed Vehicle X to exit the runway at taxiway Echo and report off the runway.  Vehicle X acknowledged and proceeded to exit; but had not reported off yet.  Meanwhile; a fellow controller asked me to ask the pilot of Aircraft X if he could say 'Hi' to a friend/fellow controller from another facility who just happened to be a passenger on Aircraft X.  I said 'Sure; will do.'   I then got the report from Vehicle X that he was off the runway and holding short.  I de-activated the RIPD; removed the 'Firetruck' from my strip bay; and I then passed the message onto Aircraft X to say 'Hi' to the passenger/controller from the one controller who was in the operating quarters at the time.  The pilot of Aircraft X politely said they would pass the message along.  After that I said 'Thank you'; issued the wind and told Aircraft X he was cleared to land Runway 13.  About 5 seconds later Vehicle Y came on ground frequency and said they had been approved to work on Runway 13 but was now quickly exiting the runway for the arriving aircraft. I replied back to FAA Mobile vehicle with 'Roger'. As soon as Vehicle Y keyed up I suddenly then remembered that I had approved him onto Runway 13 but had forgotten to give him a control instruction to exit the runway prior to clearing Aircraft X to land.  I did another visual scan of the runway to ensure there were no other vehicles/personnel on the runway and there was not.  Aircraft X landed without incident and I taxied him to the gate.  I later discussed the incident with the operator of Vehicle Y.  I apologized to him that I had forgotten about him and informed him I was going to file an report on the incident.  He told me that is why he leaves the volume on his radio turned up when he is outside of his vehicle working so he can hear transmissions.  The reason I believe this event happened was because I was not 100% focused on the task-at-hand of working the position.  I am a developmental and I am currently training on radar.  While working Local control I was writing notes regarding the training session I had just had on radar an hour earlier in the day. While there is no additional visual/audio aid that I could implement that we already are not using as a facility (i.e. firetruck on strip board & RIPD); much of it comes down to my own ability to focus on the job at hand.  By allowing myself to focus too heavily on the previous training session I was in turn not providing 100% of my attention and alertness to the position I was working.  I believe this event has forced an awareness into me that I might have become too complacent working the positions I am certified on and a need to give 100% of myself to the job where I haven't been.
MLI Tower Controller describes a situation where he almost forgets about a vehicle on the runway while an aircraft is on short final to the runway.