In 2005 I started running Tuffy liners in both of our Trek Navigators. I had talked to one of our bicycle mounted police officers about puncture protection. All of the police bikes had Tuffy liners in the tires to deal with broken glass, nails, etc. in back alleys in the city. I have run Tuffy or Slime liners in all four of our bikes since that time. When I wear out a set of tires I simply switch the liners to the new tires. With the liners we have had only one puncture flat in about 15,000 miles of riding. That was a raspberry bush thorn through a sidewall above the position of the liner.
When I pull the liners out of a worn set of tires I look at the surface of the liner against the tire. You would be amazed at what they protect against in the way of punctures.

I have been running Kevlar belted or Kevlar casing tires for several years. While puncture resistant they are not 100% puncture proof. I still run liners in them for added protection. When we are 25 to 30 miles from the car I don't want to be troubled with flats.

Several years ago we were riding through Williamsport, PA. A box store had a big shoe sale sign out. The wife's shopping gene kicked in and across the parking lot we went. As I rounded the rear of a parked car I did not see the broken beer bottle until it was too late to avoid it. The glass cut the Conti Town & Country straight across the tread and right through the casing. I stopped and looked into the cut. There was the Tuffy liner bulged up into the cut keeping the tube out of the cut which would have caused the tube to fail.

I have had flats from valve stem failures when I did not deburr the valve stem holes in rims. I had had flats when the factory rubber rim tape failed. But only one puncture flat in 7 years with the raspberry bush thorn through the sidewall.

I would point out that steel belted auto tires are not 100% puncture proof when it comes to sharp pointed objects. The early adds for the steel belted radials showed them being run over railroad tracks and axe blades but none would show them going over roofing nails.

E. Ogre