i would interpret "consistently" as meaning "always", so that the car was only instantaneously at rest and then jumped to 60mph for the whole trip. the average speed is then 60mph and the distance travelled is 60 miles. Road conditions are irrelevent as the speed is given (unless you mean hills etc and dont want to count vertical distance travelled?)Q: A car consistently travelling at 60 mph from standstill will have travelled 60 miles in one hour
Considered in isolation, i would consider this to be a mistake in the textbook. But you must remember that every course has its own style/terminology and these answers may make sense within the terminology of your course.
Have you checked the authors website for an errata page?
PS: Check you copied the question text exactly. if the question said "a car constantly accellerates to 60mph from standstill..." then the textbook answer is correct.