Interpretation ID: 3313o

Mr. Jack McCroskey
Ms. Glenda Swanson Lyle
Regional Transportation District
1600 Blake Street
Denver, CO 80202-1399

Dear Mr. McCroskey and Ms. Lyle:

This responds to your letter of September 13, 1988, asking for our advice on potential safety hazards and legal liabilities that might result from ignoring the speed restrictions on the tires used on your transit buses. You stated that your entity operates three types of bus service in the State of Colorado. The first type is a local bus, operated primarily in areas where the speed limit is 35 miles per hour (mph); the second type is an express bus, operated primarily in areas where the speed limit is 55 mph; and the third type is a regional bus, operated primarily on freeways with speed limits of 55 to 65 mph. You were interested primarily in the tires used on your express buses.

You stated that you use two types of speed-restricted tires "almost interchangeably" on the express buses. One of your speed-restricted tire types is the XT, which is speed-restricted to a maximum speed of 55 mph. The other is the DXT, which is speed-restricted to a maximum speed of 35 mph. Since the express buses are operated primarily at speeds of 55 mph, you contacted the tire manufacturer to get its advice on the acceptability of using tires that are speed-restricted to 35 mph on these buses. You enclosed copies of correspondence you received from the manufacturer, stating that its DXT and XT tires are identical, except that the DXT tire has 7/32 of an inch more undertread. The manufacturer's advice was that the tire that is speed-restricted to a maximum of 35 mph "may be used at higher speeds; but not for sustained operation." You asked whether it is advisable for your entity to continue using the tires that are speed-restricted to 35 mph on your express buses, which operate primarily at 55 mph. We strongly recommend that you not do so.

There are some notable advantages associated with speed-restricted tires, including enhanced load-carrying capabilities and greater resistance to tire damage from hitting objects in the road or curbs. However, the greater undertread on speed-restricted tires also means that the tires will generate higher temperatures at high speeds than a comparable non-restricted tire. Higher temperatures inside the tire increase the chances of a tire failure at high speeds.

NHTSA carefully considered both the advantages of speed-restricted tires and the need to ensure that such tires are properly used when it was developing Standard No. 119, New Pneumatic Tires for Motor Vehicles Other Than Passenger Cars (49 CFR 571.119; copy enclosed). Every new bus tire must be certified by its manufacturer as complying with this standard. This agency decided that Standard No. 119 should permit the continued production of speed-restricted tires, but with appropriate safeguards to ensure that these tires would not be used at higher speeds. Accordingly, Standard No. 119 specifies less stringent high speed and endurance test requirements for speed-restricted tires. Speed-restricted tires for use on vehicles other than motorcycles are exempted from the high speed performance requirements of S6.3 of Standard No. 119. This exemption reflects the fact that the tires are not designed for high speed use. For the same reason, the endurance test schedule for speed-restricted tires consists of a lower test speed and fewer total revolutions of the test wheel, as shown in Table III of Standard No. 119. Hence, the manufacturer of these speed-restricted tires has not certified that these tires comply with the performance requirements of Standard No. 119 under conditions exceeding the speed-restriction marked on the tires.

To ensure that the user of speed-restricted tires would not operate the tires at higher speeds than those at which the tires are designed to operate safely, section S6.5(e) of Standard No. 119 requires every speed-restricted tire to have the marking "Max speed mph" on the sidewall. This marking is intended to alert the tire user of the limitations of this tire, so that it will not be repeatedly used at higher speeds. Since your express buses operate primarily at speeds of 55 mph, we urge you not to equip those buses with tires labeled "Max speed 35 mph." Similarly, since your express and regional buses typically operate at speeds exceeding 55 mph on their routes, we recommend only tires without speed restrictions for these buses.

With respect to your question about potential legal liabilities in the event one of these speed-restricted tires fails while in service on one of your express buses, that is a question of State law. Since I am not familiar with the Colorado law on this subject, I must decline to offer an opinion. However, the Attorney General for the State of Colorado or other local counsel would be able to accurately advise you on Colorado's law in this area.

Sincerely,

Erika Z. Jones Chief Counsel

Enclosure

/ref:119 d:12/9/88