Thoughts that pop up in my mind are:
- At what time of the day? How far are the cities from each other? How big (in lanes) is the highway?
- Reasons for "waiting": Congestion, traffic lights / crossings, construction works...
Then you could go for, e.g.:
- simply adding up all sources of waitting time x number of cars
- Comparing capacity vs actual usage
- Average time wasted congestion per car x # cars x distance
But I think the most strucured approach would be to clarify parameters like distance and reasons for delay and then see, whether the interviewer will give away any info or detail that sounds interesting to follow up upon.