Tread Life

    Table of Contents

    Driving at lower inflation pressure impacts the rate of tread wear on tires. This will cause tires to wear out earlier than necessary and decrease tread life. When a tire is under-inflated, it puts more pressure on the shoulders of the tire and does not wear correctly. This analysis will attempt to quantify the impact of increased tread wear on consumer costs.

    Based on data provided by Goodyear (see Docket No. NHTSA-2000-8572-26), the average tread life of tires is 45,000 miles and the average cost is $61 per tire (in 2001 dollars).


    For Compliance Option 1

    Assuming a direct measurement system, the TPMS warns the driver anytime a tire is 25 percent or more below the placard and the driver inflates all of the tires back to the placard levels, then we can estimate the impact on tread life using the following calculations.

    Goodyear provided data estimating that the average tread wear dropped to 68 percent of the original tread wear if tire pressure dropped from 35 psi to 17 psi. Goodyear also assumed that this relationship was linear. Thus, for every 1 psi drop in inflation pressure, tread wear would decrease by 1.78 percent [(100-68%)/(35-17psi)]. These effects would take place over the lifetime of the tire. In other words, if the tire remained under-inflated by 1 psi over its lifetime, the tread wear would decrease by 1.78 percent or about 800 miles (45,000*0.178).

    Data from our tire pressure survey indicated that 1,575 out of 5,967 passenger car tires (26 percent) had at least one tire under-inflated by 25 percent or more below the placard level. The average under-inflation of the 4 tires for these vehicles was 6.8 psi. Based on our steady state assumptions discussed earlier, the average psi of the fleet under Compliance Option 1 would improve by 3.8 psi (placard pressure is 30 psi, steady state pressure under Compliance Option 1 is 27.0 psi, thus a 3.0 psi difference; 6.8 – 3.0 = 3.8 psi improvement). Thus, on average, passenger cars lose an estimated 3,040 miles (3.8 * 800 miles) of tread life for each tire due to the way they are currently under-inflated that could be remedied under Compliance Option 1 if everyone filled all their tires back up to the placard pressure when they were notified by a TPMS. If we assume that 90 percent of the people actually inflate their tires properly, then on average 2,736 miles of tread life would be saved per tire.

    If the average current lifetime of tires is 45,000 miles at current inflation levels, the average lifetime could be 47,736 miles with a TPMS. The agency estimates that the average lifetime per passenger car is 126,678 miles. Thus, currently the average car would have 3 sets of tires on their car over its lifetime (new, at 45,000 miles, and at 90,000 miles) and with TPMS the average car would have 3 sets of tires purchased (new, at 47,736 miles, and at 95,472 miles). The benefit to consumers is the delay in purchasing those tires and getting interest on that money at an assumed 3 percent or 7 percent rate of return. Using a mid-year 3 percent and 7 percent interest rate and discount rate, the discounted present value of these delayed tire purchases is estimated to be $5.71 at a 3 percent discount rate and $10.23 at a 7 percent discount rate for those passenger cars that would be notified by a TPMS that they are under-inflated. Since 26 percent would be notified, the present discounted benefits are $1.48 ($5.71 * 0.26) at a 3 percent discount rate,  $2.66 ($10.23 * 0.26) at a 7 percent discount rate, and 711 miles (2,736 * 0.26) of tread life.

    For light trucks, data from our tire pressure survey indicated that 1,148 of 3,950 light truck tires (29 percent) had at least one tire under-inflated by 25 percent or more compared to the placard. The average under-inflation of the 4 tires for these vehicles was 8.7 psi. Based on our steady state assumptions discussed earlier, the average psi of the fleet under Compliance Option 1 would improve by 5.2 psi (placard pressure is 35 psi, steady state pressure under Compliance Option 1 is 31.5 psi, thus a 3.5 psi difference; 8.7 – 3.5 = 5.2 psi improvement). Thus, on average, light trucks lose an estimated 4,160 miles (5.2*800) of tread life for each tire due to the way they are currently under-inflated that could be remedied if everyone filled all their tires back up to the placard pressure when they were notified by a TPMS. If we assume that 90 percent of the people actually inflate their tires properly, then on average 3,744 miles of tread life would be saved per tire.

    If the average current lifetime of tires is 45,000 miles at current inflation levels, the average lifetime could be 48,744 miles with a TPMS. The agency estimates that the average lifetime per light truck is 153,706 miles. Thus, the average light truck would have 4 sets of tires on their truck over its lifetime (new, at 45,000 miles, at 90,000 miles, and at 135,000 miles) and with a TPMS the average light truck would have four sets purchased (new, at 48,744 miles, at 97,488, and at 146,232 miles). Using the same methodology as for passenger car tires, the benefit in delaying purchasing tires is estimated to be a present discounted benefit of $23.33 at the 3 percent discount rate and $31.54 at the 7 percent discount rate. Since in 29 percent of the vehicles at least one tire is under-inflated by 25 percent or more, the average benefit for light trucks is estimated to be $6.77 ($23.33 * 0.29) at the 3 percent interest and discount rate, $9.15 ($31.54 * 0.29) at the 7 percent interest and discount rate, and 1,086 miles (3,744 * 0.29) of tread life.

    The weighted tread life savings for passenger cars and light trucks after considering current compliance for Compliance Option 1 is $4.24 [($1.48 * 8/17) + ($6.77 * 9/17)]*.99 at the 3 percent interest rate and discount rate and $6.03 [($2.66 * 8/17) + ($9.15 * 9/17)]*.99 at the 7 percent interest rate and discount rate and 900 [(711 * 8/17) + (1,086 * 9/17)]*.99) miles of tread life.


    For Compliance Options 2 and 3

    Data from our tire pressure survey indicated that 1,575 out of 5,967 passenger car tires (26 percent) had at least one tire under-inflated by 25 percent or more below the placard level. The average under-inflation of the 4 tires for these vehicles was 6.8 psi. Based on our steady state assumptions discussed earlier, the average psi of the fleet under Compliance Options 2 and 3 with direct systems would improve by 3.1 psi (placard pressure is 30 psi, steady state pressure under Compliance Options 2 and 3 is 26.3 psi, thus a 3.7 psi difference; 6.8 – 3.7 = 3.1 psi improvement). Thus, on average, passenger cars lose an estimated 2,480 miles (3.1 * 800 miles) of tread life for each tire due to the way they are currently under-inflated that could be remedied if everyone filled all their tires back up to the placard pressure when they were notified by a TPMS. If we assume that 90 percent of the people actually inflate their tires properly, then on average 2,232 miles of tread life would be saved per tire.

    If the average current lifetime of tires is 45,000 miles at current inflation levels, the average lifetime could be 47,232 miles with a TPMS. The agency estimates that the average lifetime per passenger car is 126,678 miles. Thus, currently the average car would have 3 sets of tires on their car over its lifetime (new, at 45,000 miles, and at 90,000 miles) and with TPMS the average car would have 3 sets of tires purchased (new, at 47,232 miles, and at 94,464 miles). The benefit to consumers is the delay in purchasing those tires and getting interest on that money at an assumed 3 and 7 percent rate of return. Using a mid-year 3 and 7 percent interest rate and discount rate, the discounted present value of these delayed tire purchases is estimated to be $4.68 at the 3 percent discount rate and $8.42 at the 7 percent discount rate for those passenger cars that would be notified by a TPMS that they are under-inflated. Since 26 percent would be notified, the present discounted benefits are $1.22 ($4.68 * .26) at the 3 percent discount rate,  $2.19 ($8.42 * .26) at the 7 percent discount rate and 580 miles (2,232 * 0.26) of tread life.

    For light trucks, data from our tire pressure survey indicated that 1,148 of 3,950 light truck tires (29 percent) had at least one tire under-inflated by 25 percent or more compared to the placard. The average under-inflation of the 4 tires for these vehicles was 8.7 psi. Based on our steady state assumptions discussed earlier, the average psi of the fleet under Compliance Options 2 and 3 would improve by 4.3 psi (placard pressure is 35 psi, steady state pressure under Compliance Options 2 and 3 is 30.6 psi, thus a 4.4 psi difference; 8.7 – 4.4 = 4.3 psi improvement). Thus, on average, light trucks lose an estimated 3,440 miles (4.3*800) of tread life for each tire due to the way they are currently under-inflated that could be remedied if everyone filled all their tires back up to the placard pressure when they were notified by a TPMS. If we assume that 90 percent of the people actually inflate their tires properly, then on average 3,096 miles of tread life would be saved per tire.

    If the average current lifetime of tires is 45,000 miles at current inflation levels, the average lifetime could be 48,096 miles with a TPMS. The agency estimates that the average lifetime per light truck is 153,706 miles. Thus, the average light truck would have 4 sets of tires on their truck over its lifetime (new, at 45,000 miles, at 90,000 miles, and at 135,000 miles) and with a TPMS the average light truck would have four sets purchased (new, at 48,096 miles, at 96,192, and at 144,288 miles). Using a mid-year 3 and 7 percent interest rate and discount rate, the discounted present value of these delayed tire purchases is estimated to be $18.76 at the 3 percent discount rate and $26.02 at the 7 percent discount rate for those light trucks that would be notified by a TPMS that they are under-inflated. Since in 29 percent of the vehicles at least one tire is under-inflated by 25 percent or more, the average benefit for light trucks is estimated to be $5.44 ($18.76*0.29) at the 3 percent interest and discount rate and $7.55 ($26.02 * 0.29) at the 7 percent interest and discount rate and 896 miles (3,096 * 0.29) of tread life.

    The weighted tread life savings for passenger cars and light trucks after considering current compliance for Compliance Options 2 and 3 are $3.42 [($1.22 * 8/17) + ($5.44 * 9/17)]*.99 at the 3 percent interest rate and discount rate and $4.98 [($2.19 * 8/17) + ($7.55 * 9/17)]*.99 at the 7 percent interest rate and discount rate and 740 miles [(580 * 8/17) + (896 * 9/17)]*.99) of tread life per tire.

    Table V-32 shows the tread life savings per vehicle after considering current compliance.


Table V-32
Estimated Tread Life Savings per Vehicle
(Weighted Passenger Cars and Light Trucks)
  3 % Discount Rate 7% Discount Rate
Compliance Option 1 $4.24 $6.03
Compliance Option 2 $3.42 $4.98
Compliance Option 3 $3.42 $4.98

    There are other potential non-quantified benefits of increasing tread wear. Some people would not have to purchase the last set of tires for a vehicle if they were going to scrap the vehicle soon, or if it were totaled in a crash shortly before they were going to purchase new tires. So, there will be cases where the total purchase price of tires $244 ($61 per tire * 4) will be saved. However, we can’t estimate the frequency of that occurrence.


    Property Damage and Travel Delay Savings

    Table of Contents

    Reduced stopping distance, blowouts, and loss of control in skidding will prevent crashes and reduce the severity of impacts and the injuries that result. Property damage and travel delay will also be mitigated by these improvements. To the extent that crashes are avoided, both property damage and travel delay will be completely eliminated. Crashes that still occur but at less serious impact speeds will still cause property damage and delay other motorists, but to a lesser extent than they otherwise would have.


    Preventable Crashes:

    NHTSA has developed data on the cost of both travel delay and property damage stratified by injury severity on a per-person injured basis [18]. Travel delay is defined as the value of lost time experienced by motorists not involved in a crash, but who are delayed in traffic congestion resulting from these crashes. Property damage is the value of vehicles, cargo, and other items damaged in traffic crashes. The number of injuries prevented, as well as the number of PDO crashes prevented in out of control skids, has already been estimated in Chapter V. An estimate of total PDO involved vehicles was derived as follows.

    Table V-33 summarizes the injuries that would be prevented by TPMS. To estimate the impact on PDOs, it will be assumed that PDOs are reduced in the ratio of overall occurrence of PDOs to injuries. The PDO cost data mentioned above is expressed in terms of per damaged vehicle. Therefore, PDOs will be measured in these same units. The number of vehicles involved in crashes that produced the injury savings in Table V-34 is estimated based on the average number of police reported injuries per vehicle (1.35) [19]. The results are shown on the Injury Vehicles line in that Table. For the stopping distance and blowout categories, these estimates were then multiplied by the overall ratio of PDO involved vehicles to injury involved vehicles [20] to estimate the total number of police reported PDOs that would be prevented. The out of control skidding category was handled differently because a specific estimate of PDO crashes prevented was derived in Chapter V. For this category, the number of crashes prevented (9,994) was multiplied by the overall ratio of PDO vehicles per crash [21].

    PDOs are notoriously underreported. Many localities don’t even record crashes unless they involve some variable damage threshold and often drivers involved in single vehicle PDOs will leave before police arrive. Overall, NHTSA estimates that only 52% of the vehicles involved in PDO crashes get reported[22]. An adjustment was made to the 3 PDO vehicle totals to reflect this. The final results are shown in the Total PDO Vehicles line of Table V-33.

    The MAIS0 line in Table V-33 represents uninjured occupants that are present in vehicles that avoid crashes due to TPMS. They are estimated based on the ratio of uninjured occupants to injured occupants in police reported crashes [23]. They are included here because the unit property damage and travel delay costs used in this analysis were distributed over all occupants and to fully account for all savings in these avoided crashes they must also be accounted for.


Table V-33
Property Damage and Travel Delay Savings from
Crashes Prevented by TPMS
Crash Involvements Prevented By TPMS Property Damage & Travel Delay
  Injured Persons in Crashes Due To:
Preventable
Unit Costs 
(2001)
Total
Savings
Stop. Dist Skid Flat Total
             
MAIS0 748 1997 457 3202 $1,843 $5,901,406
MAIS1 1328 3529 733 5590 $4,752 $26,566,430
MAIS2 145 393 132 670 $4,937 $3,307,512
MAIS3 62 168 32 262 $7,959 $2,085,314
MAIS4 6 16 14 36 $11,140 $401,048
MAIS5 4 10 5 19 $19,123 $363,339
Fatal 13 44 37 94 $19,974 $1,877,521
Total 1558 4160 953 6671   $40,502,570
           
Inj. Vehicles 1150 3071 704 4925    
Ratio PDO/Inj Veh 4   4      
Total P.R. PDO Veh 4589 17511 2807      
Total PDO Vehicles 8825 33675 5398 47897 $2,352 $112,657,838
             
Total Including PDOs       $153,160,408

    Table V-33 also lists the total per-case Travel Delay and Property Damage costs stratified by injury severity [24]. The costs are expressed as per- injured person for all injury levels, and per damaged vehicle for PDOs. These unit costs were multiplied by the corresponding injury and PDO incidence savings to estimate total savings in travel time and property damage from crashes prevented by TPMS.


    Non-Preventable Crashes:

    The impact on non-preventable crashes is more subtle and measuring it requires some assumptions regarding the nature of injury mitigation. The injuries prevented in non-preventable crashes are summarized in Table V-34. These represent the net impact on total injuries in each severity category after the severity of each crash was reduced. For all but minor injuries, this would typically involve a tumble-down effect, where injuries are reduced to a lower severity level rather than being eliminated entirely. Since the savings are a net result of this process, this means that the total number of injuries reduced in each category is really the sum of the savings in that category plus those injuries that tumbled-down into that category from a more severe level. To simulate this, it will be assumed that each injury mitigated will fall only one level. The second column in Table V-34 shows the resulting gross savings for each severity level. In the third column, the difference in unit costs of travel delay and property damage between the specific injury level and the next highest level are shown. These numbers represent the change in these costs that occurs from each reduction in injury levels. Total costs for each level are the product of these unit costs and the total injuries saved at that level.


Table V-34
Non-Preventable Crash Stopping Distance
Property Damage and Travel Delay Savings
  Net Injuries
Prevented
Total Injuries
Prevented
Unit Costs
(2001$)
Total
Saving
         
MAIS1 1411 1684 $2,910 $4,899,598
MAIS2 273 367 $184 $67,562
MAIS3 94 105 $3,023 $317,376
MAIS4 11 17 $3,181 $54,077
MAIS5 6 31 $7,983 $247,469
Fatal 25 25 $851 $21,263
Total       $5,607,346

    Total travel delay and property damage cost savings from non-preventable crash severity mitigation is thus estimated to total $5.6 million. Total savings from all crash types, including preventable injury and PDO crashes would total $158,767,754. Since these savings would occur over the life of the vehicle, a discount factor will be applied to express their present value. At a 3% discount rate, the present value of total travel delay and property damage savings would be $130,713,492 (.8233 combined factor). At a 7% rate, this value would be $104,786,718 (.6600 combined factor). These are the estimates for Compliance Options 2 and 3.

    For Compliance Option 1, the same methodology results in total savings from all crash types, including preventable injury and PDO crashes would total $160,871,765. At a 3% discount rate, the present value of total travel delay and property damage savings would be $133,445,724 (.8233 combined factor). At a 7% rate, this value would be $106,175,365 (.6600 combined factor).



    [18] Blincoe et al, Ibid.

    [19] Ibid.

    [20] Ibid.

    [21] Ibid.

    [22] Blincoe et al, Ibid.

    [23] Ibid.

    [24] Ibid.


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