The passenger car tire testing reported herein extends earlier published research. Results under minimally-wet conditions displayed behavior similar to those previous tests conducted at greater water depths, except for the lowest water depth considered (0.25 mm or 0.01 inches). These tests reinforce the earlier conclusion that tire tread depths of less than about 3.0 mm (4/32 inches) develop significantly less cornering friction on wet surfaces at highway speeds than will tires of greater tread depth, and can approach hydroplaning at those speeds in the presence of minimal water film thickness.
Some effects on wet friction capability due to tire aging and aspect ratio are presented, as well as some observations on the significance of tire placement. Also presented are suggested methods for relating laboratory results to real-world highways.
Estimates are reported of the number of wet-road loss-of-control cases, involving low tread depth, occurring nationally in the year 2009 (the latest year for which complete accident data were available at this writing), as an indication of the significance of tire tread depth on safety.