Tower Controller described an abort event when the local controller failed to observe the runway when clearing a departure for takeoff when a landing aircraft had not exited the runway; listing distractions and failure to adhere to facility policies as causal factors.

Narrative

A B737 landed and was instructed to turn right join the parallel taxiway and contact Ramp Control which was acknowledged by the pilot. Without visually scanning the Controller assumed the aircraft turned off at the first intersection; he hadn't. The Controller then cleared the DHC8 for takeoff; the pilot rolled and shortly later the ASDE-X issued the alarm 'Warning runway occupied'. The Controller instructed the DHC8 pilot to abort the takeoff; the pilot complied and turned off the runway approximately 2;500 short of the intersection the B737 had ultimately cleared at. At the time there were four other positions staffed including an Area Supervisor/Controller in Charge position and no one saw this event developing on the main runway even though it was not busy at the time. I believe this happened due to distractions in the work area; most notably the use of cell phones while working. The majority of Controllers; as well as Front Line Managers; use their cell phones while on position for texting; accessing the Internet (ie Facebook) and gaming. Additionally; but to a lesser degree; are extraneous conversations when traffic is slow and there are extra positions staffed to show time on position and eating meals or ordering take-out while on position. While there are policies on the use of cell phones in the operations area the obvious attitude that 'It won't happen to me' is very prevalent; and it exists with both senior and new people.