## Cars with exponentially distributed arrival times

Hi! I have a problem where cars arrive at a bridge with arrival times that are $exp(\lambda)$ distributed. If each car that arrives stays on the bridge for 1/2 unit of time, what is the probability that we have 3 or more cars on the bridge somewhere in the time interval [0,10]. I've tried to simulate this for $\lambda=1$ and the probability seems to be close to 0.49.