IAD controller described a minimal separation event involving two successive air carrier arrivals; the reporter noting a landing clearance was issued without positive assurance the prior arrival had completely cleared the runway.

Narrative

Aircraft X and Aircraft Y were on approach to Runway 19C. Aircraft X landed and due to a high speed taxiway closure rolled long and was instructed to exit the runway at Taxiway D; which is a 90 degree turn off. Aircraft Y was short final and I was watching the situation develop. I knew that for Aircraft X to be clear of the runway before Aircraft Y crossed the landing threshold Aircraft X had to keep his speed up and turn off without delay; which he was instructed to do. As Aircraft X made his turn off the runway Aircraft Y crossed the threshold. As Aircraft X cleared; I transmitted to Aircraft Y 'Aircraft Y traffic clear of the runway; cleared to land Runway 19C.' As Aircraft X was approximately 8725' down the runway and turning off the runway I judged the situation to be safe and allowed Aircraft Y to land. Looking back on the situation I was unsure if the Aircraft X aircraft was entirely clear of the runway; although it appeared so; as he did not cross the runway hold bars prior to Aircraft Y crossing the threshold. The ASDE-X was operating normally in full-core alerting and no alarm was generated. Recommendation; I always tell myself; and the developments that I instruct; to put safety above all else. In retrospect; although the aircraft had over a mile of runway separation between them; and the preceding aircraft appeared clear of the active runway; this situation could have easily been avoided by issuing go-around instructions to the arriving aircraft. Although I feel safety was never compromise and I exercised my best judgment at the time; perhaps sending the aircraft around would have been a better choice.Aircraft X and Aircraft Y were on approach to Runway 19C. Aircraft X landed and due to a high speed taxiway closure rolled long and was instructed to exit the runway at Taxiway D; which is a 90 degree turn off. Aircraft Y was short final and I was watching the situation develop. I knew that for Aircraft X to be clear of the runway before Aircraft Y crossed the landing threshold Aircraft X had to keep his speed up and turn off without delay; which he was instructed to do. As Aircraft X made his turn off the runway Aircraft Y crossed the threshold. As Aircraft X cleared; I transmitted to Aircraft Y 'Aircraft Y traffic clear of the runway; cleared to land Runway 19C.' As Aircraft X was approximately 8725' down the runway and turning off the runway I judged the situation to be safe and allowed Aircraft Y to land. Looking back on the situation I was unsure if the Aircraft X aircraft was entirely clear of the runway; although it appeared so; as he did not cross the runway hold bars prior to Aircraft Y crossing the threshold. The ASDE-X was operating normally in full-core alerting and no alarm was generated. Recommendation; I always tell myself; and the developments that I instruct; to put safety above all else. In retrospect; although the aircraft had over a mile of runway separation between them; and the preceding aircraft appeared clear of the active runway; this situation could have easily been avoided by issuing go-around instructions to the arriving aircraft. Although I feel safety was never compromise and I exercised my best judgment at the time; perhaps sending the aircraft around would have been a better choice.