Nicholas S. Johnson

Accuracy of a Damage-Based Reconstruction Method in NHTSA Side Crash Tests

Objective: Delta-V (ΔV), the magnitude of the velocity change experienced by a vehicle during a crash, is widely used as a predictor of injury risk. The National Automotive Sampling System/Crashworthiness Data System (NASS-CDS) uses the WinSMASH computer code to reconstruct ΔV based on postcrash vehicle deformation. WinSMASH, a direct descendant of CRASH3, first uses vehicle damage to estimate absorbed energy and then applies momentum conservation to estimate ΔV. This study aims to determine the accuracy of WinSMASH ΔV reconstructions for NHTSA side crash tests. Methods: This study is based upon 168 dynamic side impact tests conducted by the National Highway Traffic Safety Administration (NHTSA). For each crash test the actual ΔV for the struck vehicle was first determined from test instrumentation. WinSMASH was then used to reconstruct the struck vehicle ΔV for each test. WinSMASH-reconstructed ΔVs were compared to measured ΔVs for each test to assess reconstruction accuracy. Results: WinSMASH predicts ΔV at maximum crush, before restitution occurs. WinSMASH predictions of struck vehicle ΔV at maximum crush were 20 percent high on average when using vehicle specific stiffnesses, with a great deal of inter-case variability in the error. When compared to the total struck vehicle ΔV at separation (including restitution), WinSMASH overpredicted ΔV by 11 percent. WinSMASH overpredicted the amount of energy absorbed in collisions by 40 percent, which is consistent with overprediction of ΔV. When forced to reconstruct tests using the amount of absorbed energy calculated from the test data, error in WinSMASH ΔV effectively disappeared. Discussion: When reconstructing NHTSA side crash tests, WinSMASH overpredicts maximum crush ΔV by about 20 percent on average. The primary factors determining the amount of ΔV overprediction appear to be overprediction of absorbed energy and the assumption of zero restitution. WinSMASH vehicle side stiffness parameters are calculated based on artificially high energies; this may explain the overprediction of absorbed energy. WinSMASHs assumption of zero restitution partially masks the effect of energy overprediction. When given accurate reconstruction inputs and energy estimates, WinSMASH is capable of accurately reconstructing maximum crush ΔV in NHTSA side crash tests but cannot compensate for restitution.

Evaluation of NASS-CDS side crash delta-v estimates using event data recorders

Objective: Planar crash severity is most commonly defined by delta-V (ΔV), which is the change in a vehicles velocity vector during a crash. All ΔV estimates contained in the NASS-CDS are generated using a damage-based program called WinSMASH. WinSMASH ΔV accuracy in side crashes has not previously been validated against real-world crash data. This study will investigate the accuracy of WinSMASH ΔV estimates in real-world side crashes. Methods: This study uses biaxial ΔV data from event data recorders (EDRs) to assess the accuracy of side crash ΔV estimates in NASS-CDS crash cases. Single-event side crashes were identified in the NASS-CDS for which (a) WinSMASH ΔV had been coded and (b) biaxial EDR data were available. For these crashes, the WinSMASH-estimated resultant ΔV was compared with the EDR-recorded resultant ΔV to assess the accuracy of the former. EDR ΔV values were adjusted for an assumed average restitution value of 10 percent, based on values reported in the literature. Principal direction of force (PDOF) is the orientation of the net crash impulse relative to the vehicle and is a key parameter in WinSMASH reconstructions. NASS-CDS PDOF estimates were compared to the PDOF computed from the EDR data to assess their accuracy as well. Results: WinSMASH systematically overestimates EDR ΔV by about 12.9 percent for cars struck by cars and by about 2.4 percent for cars struck by light trucks and vans (LTVs). ΔV error was significantly different (at 95% confidence) for crashes to different areas of the vehicle side. The mean discrepancy in NASS-CDS PDOF was −0.9° with a standard deviation of 12.6°. Discussion: WinSMASH systematically overestimates ΔV at common velocity by about 13 percent for cars struck by cars and about 2 percent for cars struck by LTVs. Since WinSMASHs assumption of zero restitution is accounted for in this analysis, this suggests that WinSMASH stiffness parameters represent LTV impacts better than car impacts in side crashes. ΔV error differs by impacted area, suggesting that side impact stiffness parameters used in damage-based ΔV reconstruction must closely represent the crash mode being reconstructed for best results. NASS-CDS estimates of PDOF do not appear to exhibit any systematic discrepancy. The amount of random PDOF discrepancy observed here is consistent with prior studies.

Validation of Event Data Recorders in Side-Impact Crash Tests

This study evaluated the accuracy of 75 Event Data Recorders (EDRs) extracted from model year 2010-2012 Chrysler, Ford, General Motors, Honda, Mazda, and Toyota vehicles subjected to side-impact moving deformable barrier crash tests. The test report and vehicle-mounted accelerometers provided reference values to assess the EDR reported change in lateral velocity (delta-v), seatbelt buckle status, and airbag deployment status. Our results show that EDRs underreported the reference lateral delta-v in the vast majority of cases, mimicking the errors and conclusions found in some longitudinal EDR accuracy studies. For maximum lateral delta-v, the average arithmetic error was −3.59 kph (−13.8%) and the average absolute error was 4.05 kph (15.9%). All EDR reports that recorded a seatbelt buckle status data element correctly recorded the buckle status at both the driver and right front passenger locations. For equipped vehicles that reported side torso, side curtain, and frontal airbag deployment information, all vehicles recorded the correct status. Although only model year 2013 and later EDRs must meet Code of Federal Regulations Title 49 Part 563, seatbelt buckle status and frontal airbag deployment time are required data elements. Language: en

Improved method for roadside barrier length of need modeling using real-world trajectories

The 2011 AASHTO Roadside Design Guide (RDG) contains perhaps the most widely used procedure for choosing an appropriate length of need (LON) for roadside barriers. However, this procedure has several limitations. The procedure uses a highly simplified model of vehicle departure, and the procedure does not allow designers to specify an explicit level of protection. A new procedure for choosing LON that addresses these limitations is presented in this paper. This new procedure is based on recent, real-world road departure trajectories and uses this departure data in a more realistic way. The new procedure also allows LON to be specified for a precisely known level of protection - a level which can be based on number of crashes, injury outcomes or even estimated crash cost - while still remaining straightforward and quick to use like the 2011 RDG procedure. In this analysis, the improved procedure was used to explore the effects of the RDG procedures assumptions. LON recommendations given by the 2011 RDG procedure were compared with recommendations given by this improved procedure. For 55 mph roads, the 2011 RDG procedure appears to lead to a LON sufficient to intercept between 80% and 90% of right-side departures that would otherwise strike a hazard located 10 m from the roadway. For hazards closer than 10 m, the 2011 RDG procedure intercepts progressively higher percentages of real-world departures. This suggests the protection level provided by the 2011 RDG procedure varies with the hazard offset, becoming more conservative as the hazard moves closer to the roadway. The improved procedure, by comparison, gives a consistent protection level regardless of hazard location.

Reduction in Fatal Longitudinal Barrier Crash Rate Due to Electronic Stability Control

Electronic stability control (ESC) is a vehicle safety system designed to keep vehicles moving in the direction commanded by the driver and thereby prevent loss-of-control crashes. Previous research has shown that ESC has been highly effective at reducing road departures related to loss of control. ESC is mandatory in all U.S. passenger vehicles manufactured from model year 2012 onward; by a 2014 estimate, ESC is in approximately one-third of passenger vehicles on the road. The proliferation of ESC may therefore alter benefit-to-cost ratios for roadside barriers. The objective of this analysis was to determine the effect of ESC on fatal crashes with roadside barriers. This objective was a first step toward determining whether ESC reduced the overall rate of crashes with roadside barriers and whether ESC had any effect on impact conditions or injury outcomes in barrier crashes. For cars, ESC reduced the odds of fatal crashes with roadside barriers by about 50% and reduced the odds of fatal rollovers that occurred in association with roadside barriers by about 45%. For light trucks and vans, ESC reduced barrier fatality odds by about 40% and barrier-associated rollover fatality odds by about 55%. By 2028, when an estimated 75% of passenger vehicles will have electronic stability control, ESC will have the potential to prevent about 410 out of an estimated 1,180 possible barrier-related fatalities per year. In the long term, once installed in every U.S. passenger vehicle, ESC could prevent about 550 of those same 1,180 possible barrier-related fatalities each year.

Injury Outcome in Crashes with Guardrail End Terminals

Objective: The goal of this study is to evaluate the crash performance of guardrail end terminals in real-world crashes. Guardrail end terminals are installed at the ends of guardrail systems to prevent the rail from spearing through the car in an end-on collision. Recently, there has been a great deal of controversy as to the safety of certain widely used end terminal designs, partly because there is surprisingly little real-world crash data for end terminals. Most existing studies of end terminal crashes used data from prior to the mid-1990s. Since then, there have been large improvements to vehicle crashworthiness and seat belt usage rates, as well as new roadside safety hardware compliant with National Cooperative Highway Research Program (NCHRP) Report 350, “Recommended Procedures for the Safety Performance Evaluation of Highway Features.” Additionally, most existing studies of injury in end terminal crashes do not account for factors such as the occurrence of rollover. This analysis uses more recent crash data that represent post-1990s vehicle fleet changes and account for a number of factors that may affect driver injury outcome and rollover occurrence. Methods: Passenger vehicle crashes coded as involving guardrail end terminals were identified in the set of police-reported crashes in Michigan in 2011 and 2012. End terminal performance was expected to be a function of end terminal system design. State crash databases generally do not identify specific end terminal systems. In this study, the coded crash location was used to obtain photographs of the crash site prior to the crash from Google Street View. These site photographs were manually inspected to identify the particular end terminal system involved in the crash. Multiple logistic regression was used to test for significant differences in the odds of driver injury and rollover between different terminal types while accounting for other factors. Results: A total of 1,001 end terminal crashes from the 2011–2012 Michigan State crash data were manually inspected to identify the terminal that had been struck. Four hundred fifty-one crashes were found to be suitable for analysis. Serious to fatal driver injury occurred in 3.8% of end terminal crashes, moderate to fatal driver injury occurred in 11.8%, and 72.3% involved property damage only. No significant difference in moderate to fatal driver injury odds was observed between NCHRP 350 compliant end terminals and noncompliant terminals. Car drivers showed odds of moderate to fatal injury 3.6 times greater than LTV drivers in end terminal crashes. Rollover occurrence was not significantly associated with end terminal type. Conclusions: Car drivers have greater potential for injury in end terminal crashes than light truck/van/sport utility vehicle drivers. End terminal designs compliant with NCHRP 350 did not appear to carry different odds of moderate driver injury than noncompliant end terminals. The findings account for driver seat belt use, rollover occurrence, terminal orientation (leading/trailing), control loss, and the number of impact events. Rollover and nonuse of seat belts carried much larger increases in injury potential than end terminal type. Rollover did not appear to be associated with NCHRP 350 compliance.

Injury Risk Posed by Side Impact of Nontracking Vehicles into Guardrails

Side impact is one of the most dangerous types of guardrail crashes. Of particular concern is a nontracking vehicle sliding sideways into a guardrail end treatment. This study investigated the issue of end terminal-side crashes with the use of a data set of 142 guardrail crashes extracted from the National Automotive Sampling System–Crashworthiness Data System. Side crashes involving an end terminal were substantially over-represented in driver injuries. End terminal contact occurred in about 25% of all guardrail-side crashes but represented almost 70% of driver injuries. Terminals that were noncompliant with NCHRP Report 350 were roughly five times as likely as compliant designs to cause serious crash injury. Collisions with terminals were also about twice as likely to initiate rollover compared with collisions with the length-of-need section of the guardrail. When injuries caused by rollovers, unbelted drivers, and driver ejections were accounted for, the risk presented by terminal contact was accentuated, as was the difference between cars, light trucks, and vans in terminal impacts.