Wikipedia defines "crash incompatibility" as follows:
Crash incompatibility, crash compatibility, vehicle incompatibility, and vehicle compatibility are terms in the automobile crash testing industry. They refer to the tendency of some vehicles to inflict more damage on another vehicle (the "crash partner vehicle") in two-car crashes. Vehicle incompatibility is said to lead to more dangerous, fatal crashes, while compatibility can prevent injury in otherwise comparable crashes.
I became familiar with this issue in April of 1998 when my younger 27 year old sister was driving her car eastbound on a highway and was hit by a woman driving westbound in a Ford Explorer. The woman had lost control of her vehicle, crossed a 47 foot median, and hit my sister nearly head-on, killing her almost instantly. To my knowledge, the woman driving the Explorer suffered relatively minor injuries and has since recovered.
This prompted me to research the topic of crash incompatibility and what I found convinced me that we should be doing much more to avoid future such tragedies. Specifically, my research supported the following four points:
1998 Report on Crash Incompatibility
The overwhelming evidence was that LTVs did present a greater danger to other drivers. There was an extensive study of the aggressivity of LTVs put out by the National Highway Traffic Safety Administration in 1998 titled "The Aggressivity of Light Trucks and Vans in Traffic Crashes". It stated that "Although LTVs only account for 1/3 of all registered vehicles, traffic crashes between an LTV and any other light vehicle now account for the majority of fatalities in vehicle-to-vehicle collisions." To more precisely define the problem, it went on to measure the "aggressivity" of vehicles as defined by the following formula:
Driver Fatalities in collision partners
Aggressitivity = ---------------------------------------
Number of Crashes of subject vehicle
The analysis used statistics from the 1991—94 Fatality Analysis Reporting System (FARS) and 1991—94 General Estimates Systems (GES) and included tens of thousand of crashes. The results were as follows:
Driver Fatalities in the Struck Vehicle per 1000 Police-Reported Crashes
Sub-
Full—Size Full—Size Small Mini— Large Midsize Compact Compact
Vans Pickups SUVs Pickups vans Cars Cars Cars Cars
--------- --------- ---- ------- ----- ----- ------- ------ ------
2.47 2.31 1.91 1.53 1.46 1.15 0.70 0.58 0.45
Causes of Crash Incompatibility
The study goes on to list the reasons why LTVs are more agressive. The main reasons given were:
Concerning this last item, the study mentioned that passenger cars had to comply to a bumper standard. However, LTVs had no bumper standard and exhibited a wide variation in ride height.
2000 Report on Threat to Pedestrians
In December 2000, a report was released titled "The Emerging Threat Of Light Truck Impacts With Pedestrians". It created a Pedestrian Risk Metric (PRM) which was computed as follows:
Total Pedestrian Fatalities by Vehicle Type
PRM = -------------------------------------------
Pedestrian Accidents Involving Vehicle Type
Following are the resulting values of PRM for each 1000 such accidents:
Pedestrian Fatalities / 1000 Reported Single Vehicle-Pedestrian Impacts
Large Large Compact Large Small Mini— Passenger FARS and
Vans Pickups Pickups SUVs SUVs vans Cars GES data Source
--------- --------- ------- ----- ----- ----- --------- --------- ------
259 204 161 150 107 86 49 1995-1999 "The Emerging Threat Of Light Truck Impacts With Pedestrians"
The following is from the Conclusions of the report:
Analysis of these three databases has clearly demonstrated that pedestrians have a substantially greater likelihood of dying when struck by an LTV than when struck by a car. Furthermore, at any given speed of impact, the risk of serious injury to the head and chest has been shown to be greater in LTV impacts than in car impacts. Only for lower extremity impacts is the risk of serious injury greater for car impacts than for LTV impacts.
2003 Update on Vehicle Aggressivity
I did attempt to find later analyses of crash incompatibility to see if there had been any improvements. So as far, I have found only one study (released in May 2003) which uses the same measure of aggressivity. The following table compares the numbers from the this and the 1998 study:
Aggressivity: Deaths in other Vehicle / 1000 Police Reported Crashes with Subject Vehicle Full-Size Full-Size Small / Sub- / Large / Large Large Small Compact Mini— Large Midsize Compact Compact Mini- FARS and Pickups Vans SUVs SUVs Pickups vans Cars Cars Cars Cars Cars GES data Source --------- --------- ----- ----- ------- ----- ----- ------- ------ ------ ----- --------- ------ 2.31 2.47 --- 1.91 --- 1.53 1.46 1.15 0.70 0.58 0.45 1991-1994 "The Aggressivity of Light Trucks and Vans in Traffic Crashes" 2.93 2.70 2.05 1.51 1.44 1.04 0.85 0.77 0.60 0.41 0.39 1995-2001 "NHTSA’s Research Program for Vehicle Compatibility"As can be seen, there was little change in the agressitivity. The following is from the Conclusions of the latter report:
Light trucks and vans are continuing to increase as a percentage of the U.S. fleet. The number of occupant fatalities in cars struck by LTVs has stopped increasing in recent years. However, there still remains a significant difference in the fatality rates between LTVs and passenger cars. Large vans and large pickups are over three times more aggressive than passenger cars in all vehicle-to-vehicle crash configurations. Sport utility vehicles are over twice as aggressive as passenger cars for all vehicle-to-vehicle crash configurations.
2012 Evaluation of the Enhancing Vehicle-to Vehicle Crash Compatibilty Agreement
There was a NHTSA report release in May 2012 titled "Evaluation of the Enhancing Vehicle-to-Vehicle Crash Compatibility Agreement: Effectiveness of the Primary and Secondary Energy-Absorbing Structures on Pickup Trucks and SUVs". Its Executive Summary begins as follows:
This report analyzes the effects of the Enhancing Vehicle-to-Vehicle Crash Compatibility Agreement, generically abbreviated as EVC, which the vehicle manufacturers established in 2003 as a voluntary measure, as a means to reduce occupant fatalities of passenger cars in crashes with pickup trucks or SUVs. Specifically, the report addresses the fatality reduction due to compatibility improvements at the moment of self-certification, which varies by make and model but, according to the EVC, would be sometime up to September 2009.
Under the compatibility agreement, voluntary standards for LTVs (pickup trucks, SUVs, and vans) were agreed upon to reduce the height mismatches between these LTVs and passenger cars. These measures were specifically aimed to reduce fatalities when the front of the LTV contacts the side or the front of the car.
Hence, the compatability issue is being addressed via a voluntary agreement by the vehicle manufacturers. The Executive Summary ends as follows:
The principal finding is a statistically significant 8 percent reduction in car-occupant fatalities after light trucks self-certified to the compatibility agreement. But the results are inconsistent for pickup trucks and SUVs. The observed fatality reduction for pickup trucks is negative (-5%) and not statistically significant, while for SUVs it is a positive and statistically significant 17 percent. Furthermore, the non-parametric analysis does not show fatality reduction for significantly more than 50 percent of the makes and models. Overall, these results provide some evidence that the EVC has reduced fatalities but are not sufficiently strong to permit an unequivocal conclusion that it has been effective in reducing fatality risk to car occupants.
In the Discussion section, the report addresses a 2011 study released by the IIHS (Insurance Institute for Highway Safety) titled "Better compatibility has lessened the danger that SUVs and pickups pose to people in cars". It states:
Nevertheless, these results can be reconciled with the study recently released by IIHS. That study is strong evidence that fatality risk to car occupants in impacts by late-model light trucks has declined in absolute terms over the past decade, and, in particular, that pickup trucks and SUVs have become less aggressive over time, relative to cars. However, that study did not analytically isolate or quantify specific factors accounting for the decline. It may be due to a combination of compatibility improvements in the light trucks; crashworthiness improvements in the cars; crash avoidance technologies; and changes in vehicle mix, vehicle use, driving patterns, or the overall decline in fatality risk for all vehicles. Furthermore, the compatibility improvements in the light trucks are not limited to those involved in certifying to the voluntary standard, but could include any prior or subsequent modifications. By contrast, our results try to address exclusively the fatality reduction due to compatibility improvements close to the time of self-certification, and it is this limited effect that falls short of being unequivocally significant.
Hence, it appears that there have been some improvement in the compatability of some SUVs with cars but an improvement is much less clear regarding pickup trucks. In addition, it's not clear how incompatible LTVs still are, just that they are less so. In any event, it does seems clear that there were serious compatibility issues from at least 1991 to 2001.
The Ford Explorer
In 2002, Frontline released an interesting documentary titled "Rollover - The Hidden History of the SUV". As its title implies, it was chiefly concerned with SUV rollovers. However, it also had some interesting information regarding the Ford Explorer in 2002. Following are some points from their web site:
In what was seen as a startling admission, Ford published a report that was distributed to shareholders in which the company acknowledged that SUVs are environmentally unfriendly and can be a danger to drivers in smaller vehicles. Ford said that it made these admissions in an attempt to be more transparent, and that it was taking some steps to help address the problems. However, Ford said that it would continue to make SUVs in order to meet the strong market demand.
The link now gives an error but the Internet Archive Wayback Machine contains a version from December 16, 2001. This page does not appear to contain the steps to address the SUV problems but I did follow some links and found the information at this link. Among other things, it mentions that "The bumper beam (the impact-absorbing part) was lowered almost two inches in the 2002 Explorer to virtually the same height as the bumper on a Taurus or Sable".