1. Clarify what constitutes "passing through" and "busy". Let's define "passing through" as cars on the circle itself (not cars waiting to enter the circle). Let's define "busy" as a roundabout that constantly has cars on it. Defined thus, it doesn't matter whether it's rush hour or not, since the density of traffic won't change.
2. Clarify the time-frame -- reasonably, a day = 9am - 9pm.
3. Clarify a) the length of the roundabout circle to determine how many cars can drive on it at once; b) the average length of a car; c) the average distance between cars driving on it -- then you can determine the number of cars driving on the roundabout
4. Make a reasonable assumption about average car emissions (segmenting more / less depending on the theoretical precision you want or think the interviewer wants)...and note your units in terms of emissions/distance or emissions/time (emissions/hr would be consistent with #2, our timeframe)
5. Answer should be #2 * #3 * #4