I rarely rent cars in Europe, but when I do I follow a useful pattern. I only drive in the rural areas, I rent at the airport and drop off at another airport, avoiding heavy city traffic. When I was last in Spain in September 2013, I took the train from Barcelona to Barcelona airport and rented a car and drove all through Northern Spain without a problem, ultimately returning it at Barájas in Madrid and taking a bus to my hotel in Madrid. I took the AVE high-speed train to Toledo and back.
The freeways in Spain were in perfect condition and I only saw the Guardia Civil once.