Observing surrounding traffic is one of the primary responsibilities of a driver and research has shown that a proportion of accidents could have been prevented if the driver had reacted faster to, e.g., braking vehicles. One reason for slowed reaction is missing visual attention, used, for instance, to set up the navigation system or operate the multimedia system. This reduces driver's ability to perceive other vehicles. In this work, we propose a method to quantify the effect of missing perception of leading vehicles. We extend Time-To-Collision (TTC) to a probabilistic TTC (PTTC) model considering the (additional) perception of the driver instead of the vehicle's sensor system only. The underlying theory is that whenever the driver's gaze is off the road, he cannot observe whether vehicles ahead apply the brake. The position of leading vehicles is modeled probabilistically, according to the gaze-off-road time. Results from a naturalistic driving study (vehicle sensors, driver's face) indicate, e.g., a reduced environmental perception during lane change situations, but with little impact on road safety. Data further show that in situations with high TTC, drivers tend to operate controls in the center console. PTTC will now be used as a more realistic, driver related, TTC variant.