Parking ticket etiquette?

I went out of town for a week, and since I live in the city and park on the street, I loaned my friends my car. This was a dual favor – great for them, because they got to use my car all week, and great for me because I didn’t have to worry about my car while I was gone (you need to move your car at least once a week to avoid getting a ticket).

I just got a parking ticket in the mail from the week I was gone. I guess my friends did not realize that they had gotten a ticket (they didn’t mention anything to me). I feel awkward about approaching them to pay for the ticket. Should I just pay the ticket and not mention anything to them? Or let them know about the ticket? What is the appropriate and least awkward way to handle this?