Table of Contents

    Human Factors Issues

    The Tire Pressure Monitoring Systems (TPMS) will provide notification to drivers that their tire pressure has dropped below the level recommended by the manufacturer. However, driver response to this information may vary depending upon the nature of the information provided by the TPMS. NHTSA believes that almost all drivers will respond in some manner to the warning, but the level of information presented to the driver by different display systems may result in different behavior by drivers.

    The direct measurement systems could display individual tire pressures and tell the driver which tire(s) are low. Although individual tire pressures are not proposed to be required, this analysis assumes in Compliance Option 1 that all of the vehicles will be supplied with direct measurement systems that will display individual tire pressures because it will be helpful to drivers in terms of fuel economy, tread wear and safety. This was done because of uncertainty regarding the exact nature of displays that manufacturers will install. The indirect and hybrid measurement systems can only provide a warning lamp that tire pressure is low. Compliance Options 2 and 3 assume all vehicles will be equipped with only a warning lamp.

    We anticipate that drivers will react differently to the different amounts of information. Some drivers will keep track of the individual tire pressures and will add pressure to their tires whenever necessary, say at 10 percent below placard, even before the warning is given. These drivers will accrue more safety benefits and more benefits in terms of fuel economy and tread life than drivers that wait longer for a warning. On the other hand, some drivers who currently check their own tires frequently enough to avoid significant under-inflation may start to rely on the TPMS to indicate under-inflation, rather than checking their tires frequently and filling them up whenever they were below the placard level. We believe this would happen more often under Compliance Options 2 and 3, where only a warning lamp comes on when tire pressure goes below a specified threshold, rather than under Compliance Option 1, where individual tire pressures could be monitored continuously. These drivers would actually accrue fewer safety, tread wear and fuel economy benefits than they did without the TPMS.

    The agency has little information that would help it estimate how a TPMS would affect overall driver tire maintenance behavior. A survey question in the Bureau of Transportation Statistics Omnibus Survey of July 2001 asked 1,004 respondents "To what extent do you agree that an indicator lamp in your vehicle that warns the driver about under-inflation in any of the vehicle's tires would allow you to be less concerned with routinely maintaining the recommended tire pressure?"  The responses were 40 percent to "a very great extent", 25 percent to "a great extent", 18 percent to "some extent", 7 percent to "a little extent", and 10 percent to "no extent". Putting this information together with survey data from the tire pressure survey, where one-third of those surveyed indicated that they check their tire pressure at least once a month, indicates that some people would check their tire pressure less frequently.

    The agency has some information that would help it estimate what percent of drivers would put to use the information on individual tire pressures. From the agency's tire pressure survey, we found that about one-third of the interviewed drivers indicated that they check their tire pressure once a month or more frequently. For Compliance Option 1, we assume that one-third of the drivers would pay attention to the individual tire pressure information provided on a monitor and would refill their tires when they were 10 percent below the placard. This means that if the average passenger car tire placard is 30 psi, we assume for Compliance Option 1 that one-third of the drivers would refill their tires when they get to 27 psi. The other two-thirds of the drivers would refill their tires when the warning is given at 25 percent below placard, or 22.5 psi for the average passenger car.

    The second question is whether drivers, given a warning, will stop and inflate their tires back to the placard pressure. We do not expect driver compliance with the TPMS telltale, which is amber or yellow, to be 100 percent. In the Final Economic Assessment, we assumed that 95 percent of drivers will fill the low tire(s) to make sure they don't get a flat tire and be stranded somewhere. Given just a telltale, the driver will probably need to check all the tires. Given a reading of tire pressure on all four tires with a direct measurement system, the driver will know which tire(s) are low and need to be filled.

    This assumption was based on NHTSA's own estimates and a study relating to the Cycloid Pump. "Examining the Need for Cycloid's Pump:  An Analysis of Attitudes and a Study of Tire Pressure and Temperature Relationships", December 7, 2001 by the University of Pittsburgh Department of Mechanical Engineering, Department of Industrial Engineering. This study included a survey of people's attitudes. The survey was not a random survey of consumers representing a national picture. The 225 respondents to the survey were:

    1)      classmates, faculty, and anyone they thought would respond to an E-mail survey
    2)      a group of consumers at a supermarket who were willing to participate.

    One of the questions was:

    Q21. Would you respond to a dashboard warning lamp informing you that your tire pressure was low?

    a)      Yes
    b)      No.
    219 out of 225 (97.3%) responded Yes.

    Note that there were several questions before this one on how often do you check your tire pressure, when was the last time you checked your tire pressure, what is the recommended tire pressure in your vehicle, etc. These types of questions set up the respondents to thinking that tire pressure is an important topic worthy of checking out.

    While this is not a random sample, the question format may have biased the responses, and driver's actual deeds are often different from their telephone response, the response is overwhelming and leads some small credence to a very high estimate (our initial estimate was  95 percent of drivers will respond to a warning lamp).

    In 2003, NHTSA collected information on direct and indirect systems, in terms of tire pressure and asked the owners several questions. This report is still in progress. Preliminary results from questions in this survey to determine consumer reaction to existing TPMS systems indicated that in almost 95% of cases where vehicles had direct systems, and the driver was given a low tire pressure warning, the drivers responded by taking appropriate action. These preliminary survey results thus validate NHTSA's initial assumption. However, considering that these are all new vehicles and relatively expensive vehicles that have a direct TPMS, and that typically the reactions of purchasers of more expensive vehicles to behavioral warnings will be higher than the reactions of the average or second-time owners, we have assumed a more conservative 90 percent response rate to a warning.

    In the Final Economic Assessment we assumed that there will be a natural process whereby, people fill up their tires and then the tires lose air over time. Thus, the benefits of the system are going from the level of pressure in the tire survey to an average level of pressure between times the tires are refilled using the following assumptions:

    1. Given a warning lamp goes on, 90 percent of people will check their tires and refill them back to the placard level.
    2. Tires lose air at an average of 1 psi per month.
    3. The warning has to be given at 25 percent below placard. For passenger cars, assuming the average placard is 30 psi, the warning would be given at 22.5 psi. In Compliance Options 2 and 3, the tires would be refilled at the time of the warning, and then would slowly lose air down to 29 psi at the end of month 1, 28 psi at the end of month 2, etc, until they reached 22.5 psi again when a new warning would be given. Thus, the average steady state psi in this example is 26.3 psi [(30+29+28+27+26+25+24+23+22.5/2)/8.5].
    4. For Compliance Option 1, we assume the display that will show individual tire pressures and that one-third of the drivers would pay attention to the display and fill up their tires every time they got to 10 percent below placard or 27 psi. For these individuals that pay attention to the display, the average steady state psi in this example is 28.5 psi. We also assume that the other two-thirds of the drivers will not pay attention to the display and will fill up their tires when they get a warning at 22.5 psi. Thus, their average steady state psi is 26.3 psi. A weighted average of these is 27.0 psi (28.5*.333 + 26.3*.667).
    5. These same assumptions are used for the light truck fleet, except we assume that the average placard for light trucks is 35 psi. The following table shows the results of the steady state assumptions for the different compliance options. These mean that benefits are taken from the psi level at which vehicles would be getting a warning under each of the compliance options to the steady state assumptions of where the average fleet psi would be over time. The benefits would then be multiplied by the 90 percent response rate to get the final estimated benefits.

  Steady State psi Level
for Passenger Cars
Steady State psi Level
for Light Trucks
Compliance Option 1 27.0 psi 31.5 psi
Compliance Option 2 26.3 psi 30.6 psi
Compliance Option 3 26.3 psi 30.6 psi

    Skidding and Loss of Control

    Table of Contents

    For loss of control crashes, speed is the most critical factor. Excessive speed alone can cause a loss of control in a curve or in a lane change maneuver. Tread depth, inflation pressure of the tires, and road surface condition are the most notable of a long list of factors including vehicle steering characteristics and tire cornering capabilities that affect the vehicle/tire interface with the road. In the Indiana Tri-Level Study, under-inflation was not considered a contributing factor to a crash when there was high speed involved. It was only considered when the tires were significantly under-inflated (an undefined term generally taken by the investigators to mean at least 10 to 15 psi below recommended pressure). Still, it is hard to know whether correcting this one problem area could result in the collision being avoided or reduced in severity. That is one reason why under-inflation was never cited as the definite cause of a crash. We tried to consider this by comparing under-inflation as a percentage of all of the probable causes in crashes. Certainly, reducing under-inflation is an important area and a move in the right direction. However, it is difficult to determine what the effectiveness of increasing tire pressure would be on these crashes. The following discussions describe how inflation pressure affects these crash types to the extent known.

    Skidding and/or loss of control in a curve

    Low tire pressure generates lower cornering stiffness because of reduced tire stiffness. When the tire pressure is low, the vehicle wants to go straight and requires a greater steering angle to generate the same cornering force in a curve. The maximum speed at which an off-ramp can be driven while staying in the lane is reduced by a few mph as tire inflation pressure is decreased. An example provided by Goodyear shows that when all four tires are at 30 psi the maximum speed on the ramp was 38 mph, at 27 psi the maximum speed was 37 mph, and at 20 psi the maximum speed was 35 mph while staying in the lane. Having only one front tire under-inflated by the same amount resulted in about the same impact on maximum speed. But, the influence of having only one rear tire under-inflated by the same amount was only about one-half of the impact on maximum speed (a 1.5 mph difference from 30 psi to 20 psi).

    The agency also has run a series of tests to examine the issue of decreases in tire pressure on vehicle handling. A 2001 Toyota 4-Runner was run through 50 mph constant speed/decreasing radius circles to see the effects of inflation pressure on lateral road holding. We examined lefthand turns from 0 to 90 degrees handwheel angle for tire inflation pressures varied from 15 to 35 psi. The data indicate to us that in on-ramps/off ramps, tire inflation pressure is a critical factor in vehicle handling. The data show how much friction the vehicle can utilize, in terms of lateral acceleration (g's), before it slides off the road. The more lateral g's the vehicle can utilize, the better it stays on the road. So, if you are going around an off-ramp and need to turn the wheel 50 degrees at 50 mph, you can utilize 0.27 g's at 15 psi, or you can utilize 0.35 g's at 30 psi.

    Skidding and/or loss of control in a lane change maneuver

    In a quick lane change maneuver, under-inflated tires result in a loss of tire sidewall stiffness, causing poor handling. Depending upon whether the low tire(s) are on the front or rear axle impacts the vehicle's sensitivity to steering inputs, directional stability, and could result in a spin out and/or loss of control of the vehicle.

    Skidding and/or loss of control benefits estimate

    In Chapter IV, we estimated a target population for skidding and loss of control crashes for under-inflated tires of 247 fatalities, 23,100 injuries and 53,130 property-damage-only crashes. The agency assumes that 90 percent of drivers will fill their tires back to placard pressure.

    It is difficult to determine the effectiveness estimate, (i.e., what percent of the crashes would be avoided by just improving low tire pressure). For this analysis, we assume 20 percent effectiveness to go from a very low pressure, where a warning would be given, to the steady state condition, although it could potentially be much higher. Thus, the benefits by Compliance Option are shown in Table V-1. An example calculation resulting in the estimated 44 fatalities is (247*.90*.20*.99 to account for one percent current compliance).

Table V-1
Impacts for Skidding/Loss of Control Crashes
  PDO MAIS 1 MAIS 2 MAIS 3 MAIS 4 MAIS 5 Non-Fatal Inj.
Opt. 1 -9,468 -3,529 -393 -168 -16 -10 -4,116 -44
Opt. 2 -9,468 -3,529 -393 -168 -16 -10 -4,116 -44
Opt. 3 -9,468 -3,529 -393 -168 -16 -10 -4,116 -44

    Note that the benefits are the same for all the Compliance Options, since they all require warnings at 25 percent below placard pressure. It is assumed that the benefits would come from increasing tire pressure from a low state to a pressure close to placard pressure. This reflects the finding that the levels of under-inflation in the Indiana Tri-Level study were higher than 25 percent to have under-inflation reported as a probable cause.

Table of Contents