The inter-locking nubs are great. They link together well under extensive foot traffic. Someone commented on the metal rod sliding out... Just take it out and give it a slight bend. Reinsert it and it will stay locked in. I've used these for years and they've held up under cars and feet on all sorts of terrain. They also appear to be made from recycled rubber/plastic materials. It takes a while for the smell to dissipate, but tough construction!