The medium comes with one strap, but I'm not sure that I love it. I have considered getting a different strap that I know has been crash tested. I'm not sure if its overkill, but I am considering getting a separate seatbelt adapter to be used with it. I use a Buddy Belt with Katie, but I have considered using the PawRider seatbelt adapter to use along with the Snoozer.
PawRider Pet Seat Belt Adapter