We are building a system that can watch for certain signs of stress in drivers, specifically stress related to talking on the car phone, as may be caused by increased mental workload. To gather data for training and testing our system, subjects were asked to 'drive' in a simulator past several curves while keeping their speed close to a predetermined desired constant value. In some cases they were simultaneously asked to listen to random numbers from a speech-synthesis software and to perform simple mathematical tasks over a telephone headset. Several measures drawn from the subjects' driving behavior were examined as possible indicators of the subjects' performance and of their mental workload. When subjects were instructed (by a visible sign) to brake, most braked within 0.7-1.4 seconds after the sign came into view. However, in a significant number of incidents, subjects never braked or braked 1.5-3.5 seconds after the message; almost all of these incidents were when subjects were on the phone. On average, we found that drivers on the phone braked 10% slower than when not on the phone; additionally, the variance in their braking time was four times higher -- suggesting that although delayed driver reactions were infrequent, when delays happened they could be large and potentially dangerous. Furthermore, their infrequency could create a false sense of security. In future experiments, subjects' physiological data will be analyzed jointly with measures of workload, stress and performance.