III. TIRE PRESSURE SURVEY AND TEST RESULTS

In February 2001, the agency conducted a tire pressure study to determine the extent to which passenger vehicle operators are aware of the recommended air pressure for their tires, if they monitor air pressure, and to what extent the actual tire pressure differs from that recommended tire pressure by the vehicle manufacturer on the placard. The most useful information for this analysis is the snap shot in time that tells us where the actual tire pressure of the fleet is in comparison to the vehicle manufacturer's recommended tire pressure. Although this was not a nationally representative survey, it is being treated as such in this analysis.

The field data collection was conducted through the infrastructure of 24 locations of the National Automotive Sampling System Crashworthiness Data System (NASS CDS). Data were collected on 11,530 vehicles that were inspected at a sample of 336 gas stations. There were 6,442 passenger cars, 1,874 sport utility vehicles (SUVs), 1,376 vans, and 1,838 light conventional trucks. Data can be separated by passenger cars with P-metric tires; trucks, SUVs and vans with P-metric tires; and trucks, SUVs, and vans with either LT-type or high flotation tires. For this analysis we only compare the passenger car tire pressures and the light truck tire pressures, without separating the light trucks by type of tire. Complete data were collected on 5,967 passenger cars and 3,950 light trucks for a total of 9,917 vehicles.

The average placard pressure for passenger cars was about 30 psi, while the average placard pressure for light trucks was about 35 psi, although the light trucks have a much wider range of manufacturer recommended placard pressure.

The issue addressed is how often drivers would get a warning from a low tire pressure monitoring system. Several scenarios were examined, as shown in Table III-1:

• Assume the driver would be warned anytime one or more tires fell 20%, 25%, or 30% below the placard recommended pressure, assuming a direct measurement system
  
• Assume the driver would be warned anytime one or more tires fell 6 psi (or 10 psi) below the placard recommended pressure, assuming a direct measurement system

Because of the wide range of placard pressure for light trucks, it was determined that it would be best to propose a percentage reduction from the placard than a straight psi reduction. For Alternative 1, an average of 38 percent of the passenger car and light truck drivers in the tire pressure survey would get a warning with a direct measurement system that activated at 20 percent or more below the placard pressure.

Table III-2 (a) shows, for example, the distribution of tire pressure when at least one tire is 20 percent or more below placard in terms of whether one, two, three, or all four tires were at least 20 percent below placard. Tables III-2 (b) and (c) show similar results for 25 percent and 30 percent below placard. The upgraded indirect measurement systems that work on relative wheel speed would not be able to pick up when all four tires have lost air at about the same rate.

Table III-3 shows that the tires on the rear axle are more likely to have a larger gap between actual tire pressure and the recommended level on the placard.

Table III-4 provides an analysis of what percent of the drivers would get a warning with an indirect measurement system that compares relative wheel speed of the four wheels. An assumption was made that if wheel speed were measured in all four wheels (an upgrade for some vehicles), then a comparison of wheel speed could be made for all situations except when all four tires lose air at about the same rate. For analytical purposes we used from our tire pressure survey (maximum tire pressure minus the minimum tire pressure) divided by the maximum tire pressure to get an average reduction. The maximum tire pressure was used as the denominator since supposedly we are starting at placard tire pressure and decreasing tire pressure from there. Since the indirect systems use a relative measurement, it cannot tell whether the tire pressure is over placard or under placard. For the benefit analyses done in this assessment, cases were not considered in which there were a relative differential in tire pressure of 25 percent or more, yet none of the tires were below placard. Thus, for example, if placard pressure was 30 psi, and the four tire pressures were 30, 30, 30, and 60 psi, this case was not included in the benefit calculations. For Alternative 2, an average of 24 percent of the passenger car and light truck drivers in the tire pressure survey would get a warning with an indirect measurement system that activated at 25 percent or more differential in wheel speed.

The current indirect measurement systems (which can determine relative differential in wheel speed of about 30%), give a warning less than 19 percent of the time. For this scenario, we use "less than 19 percent of the time", since the current systems do not always provide a warning when two tires are high and two tires are low in pressure. Without knowing the various algorithms used by the manufacturers, this estimate could not be pinpointed closer.

In summary, based on the tire pressure survey the agency conducted:
Alternative 1: a direct measurement system would result in 38 percent of the light vehicles operators being notified of low tire pressure.
Alternative 2: an upgraded indirect ABS-based measurement system would result in 24 percent of the light vehicles operators being notified of low tire pressure. The current indirect ABS-based measurement systems being used today would result in less than 19 percent of the light vehicles operators being notified of low tire pressure. [Note that low tire pressure is defined differently for each system.]

Table III-1
Percent of Vehicles That Would Get a Warning
Assuming a Direct Measurement System

Table III-2 (a)
Distribution of the Number of Tires on Vehicles
That Have One or More Tires that are
20% or more Below Placard

Table III-2 (b)
Distribution of the Number of Tires on Vehicles
That Have One or More Tires that are
25% or more Below Placard

Table III-2 (c)
Distribution of the Number of Tires on Vehicles
That Have One or More Tires that are
30% or more Below Placard

Table III-3
Front versus Rear Axle Differences
Vehicles with one or more tires below placard

Table III-4
Percent of Vehicles That Would Get a Warning
Assuming an Indirect Measurement System

TPMS Test Results

The agency tested six direct measurement systems to determine both the level at which they provided driver information and the accuracy of the systems. The warning level thresholds were determined by dynamic testing at GVWR at 60 mph by slowly leaking out air to a minimum of 14 psi. Some of the systems provide two levels of driver information, an advisory and a warning level. System F was a prototype with much lower thresholds for advisory and warning than the other systems. If System F is not considered, the typical advisory level is given at 20 percent under placard pressure, while the warning level averaged 36 percent below the placard. The static accuracy tests showed that those systems that displayed tire pressure readings were accurate to within 1 to 2 psi.

Table III-5
Direct measurement systems
Driver information provided at (%) below placard

The agency tested four indirect measurement systems to determine when they provided driver information. The warning thresholds were determined by slowly leaking out air to a minimum of 14 psi, while driving at 60 mph under a lightly loaded vehicle weight condition (LLVW) and at gross vehicle weight rating (GVWR). Table III-6 provides these results. The agency believes that the difference in the warning levels between the front and rear axle are due to variability in the system.

Table III-6
Indirect measurement systems
Driver warning provided at (%) below placard

Vehicle Stopping Distance Tests

One of the potential safety benefits the agency is examining is the impact of low tire pressure on vehicle stopping distance. Two sets of data are available from different sources - Goodyear Tire and Rubber Company and NHTSA's Vehicle Research and Test Center (VRTC). The information provided by these sources do not lead to the same conclusions.

Table III-7 shows data provided by Goodyear on an ABS vehicle. These wet stopping distance data indicate:

1. Stopping distance generally increases with lower tire pressure. The only exception was on concrete at 25 mph.

2. With fairly deep water on the road, (0.050 inches is equivalent to 1 inch of rain in an hour) lowering inflation to 17 psi and increasing speed to 45 mph increases the potential for hydroplaning and much longer stopping distances.

3. Except for 25 mph on macadam, the difference between 25 and 29 psi is relatively small.

Goodyear provided test data to the agency on Mu values to calculate dry stopping distances. This information is used in the benefits chapter later in this assessment.

Table III-7
Braking Distance (in feet) provided by Goodyear
Wet Stopping Distance (0.050" water depth)

Table III-8 shows test data from NHTSA - VRTC on stopping distance. Tests were performed using a MY 2000 Grand Prix with ABS. Shown is the average stopping distance based on five tests per psi level. The concrete can be described as a fairly rough surface that has not been worn down like a typical road. The asphalt was built to Ohio highway specifications, but again has not been worn down by traffic, so it is like a new asphalt road. A wet road consists of wetting down the surface by making two passes with a water truck, thus it has a much lower water depth than was used in the Goodyear tests.

Table III-8
Braking Distance (in feet) from NHTSA testing
Stopping Distance from 60 mph

These stopping distances indicate:

1. There is an increase in stopping distance as tire inflation decreases from the 30 psi placard on this vehicle on both wet and dry concrete.

2. On wet and dry asphalt, the opposite occurs, stopping distance decreases as tire inflation decreases from the 30 psi placard.

3. There is very little difference between the wet and dry stopping distance on the concrete pad (about 4 feet at 30 psi), indicating the water depth was not enough to make a noticeable difference on the rough concrete pad. There is a larger difference between the wet and dry stopping distance on the asphalt pad (13 feet at 30 psi).

4. No hydroplaning occurred in the NHTSA tests, even though they were conducted at higher speed (60 mph vs. 45 mph in the Goodyear tests) and at lower tire pressure (15 psi vs. 17 psi in the Goodyear tests). Again, this suggests that the water depth in the VRTC tests was not nearly as deep as in the Goodyear testing.

In general, these data suggest that the road surface and depth of water on the road have a large influence over stopping distance. Given a specific road condition, one can compare the difference in stopping distance when the tire inflation level is varied. The Goodyear test results imply that tire inflation can have a significant impact on stopping distance, while the NHTSA testing implies these impacts would be minor or nonexistent on dry surfaces and wet surfaces with very little water depth.