I'm guessing that the answer they're looking for here is that as the speed increases the wind resistance increases more. This means that if you want to go 10 % faster maybe your fuel consumption will increase with 15%. Staying below the speed where wind resistance really begins to hurt lets you get away with using less fuel, which is better for the environment.
30 feet
30 mph, as a mile is longer than a kilometer.
It will take about 30 minutes of total driving time at a constant 30 mph. Add some time for stops or delays.
At zero spedd , no miles. at 30 mph = 30 miles At 60 mph 60 miles.
Most cars can stop from 60 mph in about 100 feet or so. If the relationship is linear then a car should stop from 30 mph in about 50 feet.