I have a 40 lb. Australian Shepherd who injured her knee a few months back. Once an excellent jumper, with her injury, my dog can no longer jump, or even climb stairs. I have a PetStep ramp I had purchased several years ago, for another larger dog, so I brought it out and started using it on the back steps for my injured aussie.  The PetStep ramp has 15 1/2 inches of space between the rails allowing for a safer climb and less chance of a missed step. The molded raised slats with rubberized texture gives the surface the best traction for the safest and easiest climb, and the ramp is the longest pet ramp I have found to date.