A queue of slow moving traffic is 5 miles long. It takes 15 minutes to pass a particular road sign. A police car takes a total of 20 minutes driving at constant speed to travel from the back of the queue to the front and return to the back. How fast does the police car travel?

I have tried this question in many ways, including graphs and algebra, but I just cant find the speed. There seem to be far to many unknowns. Please help! Thanks in advance