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Published on September 4, 2006

Review of tread depth, placement for wet traction

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Date Published September 4, 2006

Although it generally is accepted that the most significant safety concern on the highway is the ``other driver''1, tires probably are the most important vehicle component from a safety standpoint.

All control forces to the automobile are transmitted through the tires, and all other applied forces, such as gravity, inertia and aerodynamic, are reacted at the tires. Collision forces are the only exception.

Water depth and road surface texture are important factors in wet-road friction, but are beyond the control of the vehicle owner-operator. Vehicle speed and tire tread depth, as well as tire pressure, are in his or her control.

Clearly, the easiest and safest response to wet road conditions is to reduce speed, but, except for during heavy downpours, where vision is restricted, this generally does not occur. Significant tire-roadway friction reduction occurs even on minimally wet roads. Low tire pressure can have a significant effect on handling, but is not addressed here.

No national tread depth requirement is in place, although NHTSA has established inspection standards for tires (49 CFR 570.9(a) and 570.62(a)).

Most states in the U.S. require a minimum of 2/32 of an inch tread depth on passenger car tires2 (See appendix), and much of the European Union requires 1.6 mm (the metric equivalent to 2/32 of an inch).3

A common tread wear bar height requirement of 2/32 of an inch (1.6 mm) exists for both the U.S. (FMVSS No. 109) and EU markets.

No rational basis relating to roadway safety has been found for the minimum tread depth requirement of 2/32 of an inch (1.6 mm).

Research from as early as 19704 has addressed the relationship of tread depth to wet traction. Investigations in the 1980s continued to focus on this issue.5, 6

More recently, tire tread depth versus highway water depth and speed has been addressed, both with regard to tire-roadway frictional capabilities and also with regard to placement of tires with different tread depths and their effect on vehicle stability on wet roads.7

NHTSA was petitioned in 1995 to increase the 2/32 of an inch inspection standard for passenger cars for reasons other than wet traction. That petition was denied.8

Research results

The friction data presented in what follows are from research reported in Blythe and Day.7 These data are the result of laboratory tests performed at the Calspan Tire Research Facility (TIRF).7 The test tires were Michelin Symmetry, P215/75R15.

The flat bed tire test machine surface is an 80 grit polycut material manufactured by 3M Corp. Test results are valuable for comparison of effects of parameter changes but must be scaled properly for application to any particular road surface.

The data on vehicle stability resulting from various tire tread placements are the results of simulations and field tests.7

Friction vs. tread, water and speed

Fig. 1 gives longitudinal peak friction results for the minimum water depth tested (0.05 inches above the asperities). Results for greater water depths, and for sliding friction, have been reported.7

Significant reductions in friction can be noted for speeds greater than about 60 mph and tread depths less than 4/32 of an inch. For all practical purposes, friction values less than 0.1 essentially represent hydroplaning.

Fig. 2 gives lateral peak friction results for the minimum water depth tested. Although generally slightly higher than longitudinal friction, the results are very similar, with similar conclusions drawn.

Fig. 3 recasts the data to show longitudinal peak friction versus tread depth directly for the minimum water depth of 0.05 inches. This graph shows clearly the benefit of a tread depth of 4/32 inches over 2/32 inches at speeds more than 60 mph.

Fig. 4 illustrates the effect of water depth on longitudinal peak friction for a tread depth of 4/32 inches. At water depths of more than 0.10 inches, even that tread depth does not provide sufficient friction at 60 mph.

Fig. 5 compares longitudinal and lateral peak friction over a range of speeds and for tread depths of 2/32 inches and 4/32 inches. The poor friction capability for tires with 2/32 inches tread, on this minimally wet surface, at speeds of 60 mph and greater, is obvious.

Tire placement, vehicle stability

Some members of the scientific community have known for many years (at least from 1970) that the best rubber always should be on the rear wheels of passenger cars, to provide for stability under wet-road conditions.9 In 1982 the Tire Industry Safety Council stated ``if selecting only a pair of replacement tires... put the two new tires on the rear wheels for better handling''10 More recently, the Tire Industry Association, in recommending that, when only two tires are replaced, the two new tires should be on the rear, included the phrase `` prevent an over-steer condition or loss of vehicle stability...''11 The Rubber Manufacturers Association recommends that ``...newer tires should be installed on the rear axle unless the new replacement tires are of a lower speed rating...''12 This caveat takes into account that the lower-speed rated tires will have lower critical characteristics than the higher-speed tires. The fundamental consideration is to have the tires with the best traction and lateral stiffness properties on the rear.

Even with this knowledge, it is not unusual today to find tire installers insisting that the best rubber should be on the front wheels.

Fig. 6 shows the results of three-dimensional simulations when the best-treaded tires are on the front and on the rear wheels of a 1990 Ford Thunderbird Super Coupe. The simulations were carried out using the Engineering Dynamics Corp. Vehicle Simulation Model, part of the HVE simulation environment.13

The best-treaded tires had about 10/32 inches and the poor tires had about 1/32 inches of tread depth, so this represents a rather extreme case.7 Classical under-steer and over-steer are easily identified.


1. No federal minimum tire tread depth requirement exists at the present time. Because tires are an obvious part of interstate commerce, a federal requirement should be considered.

2. No rational basis in highway safety has been found for the present legal minimum tread depth requirements or wear bar height requirements in both the U.S. and the EU.

3. The present legal minimum tread depth requirements do not provide adequate tire-wet roadway friction at highway speeds.

4. The common practice of mounting two new tires on the front wheels, while leaving worn tires on the rear, can lead to producing an over-steering vehicle on wet roads, with the resulting potential for loss of stability.

5. It is recommended that consideration be given to increasing the legal minimum tread depth requirement for passenger cars from 2/32 of an inch (1.6 mm) to a value closer to that suggested by recent research (4/32 of an inch or approximately 3 mm), and also increasing the treadwear bar height, over an appropriate period of time, to accommodate the necessary industry adjustments. Such a change will not only increase minimum wet traction capability to values more consistent with safety requirements, but also will positively affect tire late-life durability concerns.

6. It is recommended that greater publicity and training be given relative to the placement of new tires, when only two new tires are being installed, so that those new tires always are installed on the rear wheels of a passenger car.


1. ``Traffic Safety'' by Leonard Evans, published by Science Serving Society, Bloomfield Hills, Mich., 2004. Pp. 413, 414.

2. ``Legal Minimum Tread Depth for Passenger Car Tires in the U.S.A. - a Survey'' by William Blythe and Debra E. Seguin, Traffic Injury Prevention, Taylor & Francis Publishers, June 2006.

3. ``Tyres, Road Surfaces and Reducing Accidents: a Review'', by John C. Bullas, published by the County Surveyor's Society and the Foundation for Road Safety Research, U.K. May 2004. Pp. 45-50. (Available at and

4. ``The Effect of Tread Pattern Depth on Skidding Resistance,'' Road Research Laboratory, Report LR 323, Ministry of Transport, U.K. 1970.

5. ``Influence of Tread Wear Irregularity on Wet Friction Performance of Tires'', in ``Frictional Interaction of Tire and Pavement,'' ASTM STP 793, W. E. Meyer and J. D. Walter, Eds., American Society for Testing and Materials, pp. 41-64, 1983.

6. ``Tyre Technology'' by T. French. Adam Hilger, Publisher, Bristol and New York. ISBN 0-85274-360-2. 1988. Pp. 67-70.

7. ``Single Vehicle Wet Road Loss of Control; Effects of Tire Tread Depth and Placement'' by William Blythe and Terry D. Day. Society of Automotive Engineers Paper 2002-01-0553, SAE International, Warrendale, Pa., March 2002.

8. Product Safety and Liability Reporter, (1996) The Bureau of National Affairs Inc., Washington, D.C.

9. ``Tire Use Survey: The Physical Condition, Use and Performance of Passenger Car Tires in the United States of America'' by J. L. Harvey and F. C. Brenner. Technical Note 528, National Bureau of Standards, Washington, D.C., May 1970.

10. ``New Consumer Tire Guide'' Tire Industry Safety Council, Washington, D.C. 1982.

11. ``200 Level Basic Training Series'' Module Three, Tire Replacement Guidelines, p. 3-11. Tire Industry Association. 2003.

12. ``Replacement Guidelines for Passenger and Light Truck Tires'' Rubber Manufacturers Association. January 2005.

13. ``Validation of the EDVSM 3-Dimensional Vehicle Simulator'' by Terry D. Day. SAE Paper No. 970958, February 1997.

Technical Notebook edited by Harold Herzlich

* * *


Table 1 (Reprinted from Blythe and Seguin, 2006, Reference 2, with approval)


Alabama: 2/32'': Alabama Motor Vehicle Code Title 32-5-210

Alaska: 2/32'': 13 AAC 04.230.(e) Tires

Arizona: 2/32'': Arizona Revised Statute/Criminal and Traffic Law Manual 28-928

Arkansas: none: Arkansas Vehicle Code 27-14-215, 27-49-214, 27-37-401

California: 1/32'': Vehicle Code Section 27465

Colorado: 2/32'': Colorado Revised Statutes 42-4-228

Connecticut: 2/32'' : Vehicle Code Section 14-98a-4

Delaware: 2/32'': Delaware Criminal and Traffic Law Manual Title 21

Florida: 2/32'': Federal Motor Vehicle Safety Standards 109, S4.21(d)

Georgia: 2/32'': Georgia Law Enforcement Code 40-8-74

Hawaii: 2/32'': Periodic Motor Vehicle Inspection Rules 19-133.2

Idaho: 1/32'': Vehicle Inspection Manual 9-2

Illinois: 2/32'': 625ILCS 5/12-405.5

Indiana: 2/32'': Indiana Code 9-19-18/defers to Code of Federal Regulations Title 49

Iowa:2/32'': DOT/Iowa Vehicle Code 393.75(c)

Kansas: 2/32'': Kansas Vehicle Law Book 2002/Statutes Annotated Chapter 8-1742(e-3)

Kentucky: 2/32'': Kentucky Administrative Regulations Title 601 & 603/defers to CFR Title 49 paragraph 393.75(c)

Louisiana: 2/32'': Motor Vehicle Inspection Requirements for Passenger Vehicles Only, Page 2

Maine: 2/32'': Inspection Manual/Motor Vehicle Statutes Title 29A Sec. 1917

Maryland: 2/32'': Code of Maryland Regulations Title 11 Subtitle 14 Chapter 02, Section .04

Massachusetts: 2/32'': Motor Vehicle &Traffic Laws of Mass. (10) (a) 2

Michigan: 2/32'': Michigan Vehicle Code 257.710 (iii)

Minnesota: 2/32'': Minnesota Statute 169.721

Mississippi: 2/32'': Mississippi defers to Code of Federal Regulations Title 49 paragraph 393.75(c)

Missouri: 2/32'': Code of Regulations 11 CSR:50/defers to CFR Title 49 paragraph 393.75 (c)

Montana: none: Montana Codes Annotated 61-9:Subsection 4

Nebraska: 2/32'': Nebraska DMV-Vehicle Code 60-6, 252 (c)

Nevada: 2/32'': Nevada Administrative Code: Traffic Laws NAC 484-205

New Hampshire: 2/32'': Motor Vehicle Code 266

New Jersey: 2/32'': New Jersey Statutes Title 39 Chapter 3, section 72

New Mexico: none: New Mexico Criminal and Traffic Law Manual Chapter 66, Article 3, section 801 66-3-901 66-847

New York: 2/32'': NY State DMV-Division of Vehicle Safety Services

North Carolina: 2/32'': DOT/North Carolina State Law 20-122.1

North Dakota: none: North Dakota Century Code 39-21

Ohio: 2/32'': Ohio Administrative Code/Unsafe Vehicle Statute 4501:2-1-06

Oklahoma: 2/32'': Oklahoma Administrative Code 595:20-5-22

Oregon: 2/32'': DMV/DOT ORS 815.010 and 815.030 Administrative Rule 735-100-0010

Pennsylvania: 2/32'': DOT/Penn. Vehicle Equipment and Inspection Regulations

Rhode Island: 2/32'': Rhode Island General Law Title 31 Chapter 31-23 Section 31-23-45

South Carolina: none: South Carolina Dept. of Public Safety Title 56

South Dakota: 2/32'': SDCL 32-19-13

Tennessee: 2/32'': Tennessee defers to Code of Federal Regulations Title 49

Texas: 2/32'': Vehicle Inspection Rules and Regulations Manual 20.28-2 (h)

Utah: 2/32'': Utah Criminal and Traffic Code Chapter 41-Article 6-Section 150

Vermont: 2/32'': Vermont Periodic Inspection Manual Section 2-A-1

Virginia: 2/32'': Code of Virginia Title:: 46.2-1043

Washington: 2/32'': Revised Code of Washington 46.37.425

Washington, D. C.: 2/32'': Vehicle Inspection Handbook Set/1999 Chapter 2

West Virginia: none: West Virginia Code 17C-15-37

Wisconsin: 2/32'': Wisconsin Administrative Code, Transportation 305.30

Wyoming: 2/32'': Wyoming Statute 31-5-956

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