Under-inflation affects many different types of crashes. These include crashes which result from:
The agency has quantified the effects of under-inflation in a crash involving skidding and loss of control, flat tires and blowouts, and the reduction in stopping distance. However, it cannot quantify the effects of under-inflation on hydroplaning and overloading the vehicles. The primary reason that the agency can't quantify these benefits is the lack of crash data indicating tire pressure and how large of a problem these conditions represent by themselves, or how often they are contributing factors to a crash. The agency has just starting collecting tire pressure in its crash data investigations.
Skidding and/or loss of control from hydroplaning
The conditions that influence hydroplaning include speed, tire design, tread depth, water depth on the road, load on the tires, and inflation pressure. At low speeds (less than about 50 mph), if your tires are under-inflated, you actually have more tire touching the road. However, hydroplaning does not occur very often at speeds below 50 mph, unless there is deep water (usually standing water) on the road. As you get to about 55 mph and the water pressure going under the tire increases, an under-inflated tire has less pressure in it pushing down on the road and you have less tire-to-road contact than a properly inflated tire as the center portion of the tread gets lifted out of contact with the road. As speed increases to 70 mph and above and water depth increases due to a severe local storm with poor drainage, the under-inflated tire could lose 40 percent of the tire-to-road contact area compared to a properly inflated tire. The higher the speed (above 50 mph) and the more under-inflated the tire is, then the lower the tire-to-road contact and the higher is the chance of hydroplaning.
Tread depth has a substantial impact on the probability of hydroplaning. If you make a simplifying assumption that the water depth exceeds the capability of the tread design to remove water (which most likely would occur with very worn tires), then an approximation of the speed at which hydroplaning can occur can be estimated by the following formula:
Hydroplaning speed = 10.35 x inflation pressure 
Under this assumption of water depth exceeding the capability of the tread design to remove water:
At 30 psi, hydroplaning could occur at 56.7 mph
At 25 psi, hydroplaning could occur at 51.8 mph
At 20 psi, hydroplaning could occur at 46.3 mph.
This is presented to show the relative effect of inflation pressure on the possibility of hydroplaning.
Overloading the vehicle
When a vehicle is overloaded, (too much weight is added for the suspension, axle, and tire systems to carry) and the tires are under-inflated, there is an increased risk of tire failures. This can result in a loss of control of the vehicle.
Potential Benefits for Antilock Brake Systems
If a manufacturer decided that the difference in the cost between an indirect and direct TPMS was enough to make antilock brakes a marketable feature for that vehicle, then it might decide to increase its use of ABS and use an indirect TPMS to meet the phase-in part of the final rule. The agency has been analyzing the safety impacts of ABS for several years. The initial findings  were mixed. Fatal crash involvements in multi-vehicle crashes on wet roads and fatal crashes with pedestrians and bicyclists were significantly reduced. However, these reductions were offset by a statistically significant increase in the frequency of single vehicle, run-off-road crashes (rollovers or impacts with fixed objects). The run-off-road crashes were surprising in view of the good performance of ABS in stopping tests conducted by the agency and others. The agency has spent several years trying to determine why run-off-road crashes have increased with ABS, without a satisfactory answer.
Two more recent studies of ABS have found no statistically significant fatality improvement with ABS. The Farmer study from IIHS  found the results shown in Table V-35. (A ratio of 1.0 means there is no effect on fatalities. Less than one is a reduction in fatalities, more than 1.0 is an increase in fatalities. In order for the results to be statistically significant, the confidence bounds would have to be both below 1.0 or both above 1.0). The only statistically significant findings were that fatalities went up in non-GM cars in calendar years 1986-1995 and overall from 1986-1998.
|All crashes||95 percent confidence bounds|
|GM cars in 1993-95||1.03||.94||1.12|
|GM cars in 1996-98||.96||.87||1.05|
|GM cars in 1993-98||.99||.93||1.05|
|Non-GM cars in 1986-95||1.16 (Significant)||1.06||1.27|
|Non-GM cars in 1996-98||.91||.77||1.06|
|Non-GM cars in 1986-98||1.09 (Significant)||1.01||1.18|
Farmer's theory is that people learned how to use ABS better in calendar years 1996-98 and they were no longer overinvolved in run off the road fatal crashes. Farmer never states that ABS reduced fatalities. His statement on the GM cars for 1996-98 is "When all fatal crash involvements were considered, disregarding in which vehicle the fatalities occurred, the risk ratio was slightly lower than, but not significantly different from 1.0."
The second recent analysis by Ellen Hertz (NHTSA) , in which she included optional ABS to get more cases, also resulted in no overall statistically significant findings for fatalities. ABS effects were examined separately for passenger cars and light trucks for five types of crashes (frontal impacts, side impacts, rollover, run-off-road, and pedestrian). The only statistically significant finding was that fatalities in light truck rollover crashes went up in ABS vehicles compared to non-ABS vehicles (see Table V-36). In this study, a negative is an improvement in safety (fewer fatalities) and a positive is an increase in fatalities.
|Point||95 percent confidence bounds|
|Frontal – PC||-4.9%||-19.9%||11.5%|
|Frontal – LTV||17.9||-7.1||49.6|
|Side Impact – PC||32.4||-1.0||77.2|
|Side Impact – LTV||-0.3||-42.3||72.2|
|Rollover – PC||12.3||-17.2||52.2|
|Rollover – LTV||106.5 (Significant)||49.2||185.9|
|Run-Off-Road – PC||-13.4||-28.1||4.2|
|Run-Off-Road – LTV||21.8||-12.6||69.5|
|Pedestrian – PC||-0.4||-16.3||18.4|
|Pedestrian – LTV||-22.7||-50.1||19.6|
The Hertz study did find that antilock brakes had an overall effect of reducing crashes, but not fatalities.
If NHTSA believed that antilock brakes were cost/beneficial, we would consider requiring them to be installed. We have not considered requiring antilock brakes because we have not been able to show that they are beneficial in reducing fatalities. Reducing their costs, by offsetting the costs with a TPMS, does not affect our conclusions to date that we have not been able to prove that ABS reduces fatalities.
 "Mechanics of Pneumatic Tires" edited by Samuel K. Clark of the University of Michigan, published by NHTSA, printed by the Government Printing Office in 1981.
 "Preliminary Evaluation of the Effectiveness of Antilock Brake Systems for Passenger Cars", NHTSA, December, 1994, DOT HS 808 206.
 "New Evidence Concerning Fatal Crashes by Passenger Vehicles Before and After Adding Antilock Braking System", Charles M. Farmer, Insurance Institute for Highway Safety, February, 2000.
 "Analysis of the Crash Experience of Vehicles Equipped with All Wheel Antilock Braking Systems (ABS) – A Second Update Including Vehicles with Optional ABS", NHTSA, September 2000,
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