History and Background of SMAC
In 1952, a pioneer program in highway safety research, the Automobile Crash Injury Research Program (ACIR), was created with the objective of determining injury causation among occupants of cars involved in accidents, in order that the injuries might be prevented or mitigated through improved vehicle design. By the mid sixties, 31 states had participated in the program and provided over 50000 cases for study [1]. The main criterion for classifying severity in the ACIR program was through the use of comparison pictures of damaged vehicles.
Also during the 60's, the digital computer came of age. Mainframe computers, which filled entire floors of buildings, cost hundreds of thousands to millions of dollars evolved into time-sharing, batch processing machines. These were used in conjunction with 9-track tapes, card punch machines and terminals to provide to scientists, engineers and others number crunching capabilities unlike any utility ever before imagined. The digital computer quickly became an integral part of scientific research and development.
In September 1966, President Lyndon Johnson signed the National Traffic and Motor Vehicle Safety Act and the National Highway Safety Act. These established the authority to develop both the Federal Motor Vehicle Safety Standards and the National Traffic Safety Agency (currently known as the NHTSA). As part of signing the legislation President Johnson stated that "auto accidents are the biggest cause of death and injury among Americans under 35". In 1965, 50,000 people were killed on the nation’s highways in auto accidents.
The SMAC computer program was initially created as a feasibility study by researchers at Cornell Aeronautical Lab (currently known as Calspan). The researchers at Cornell were interested in demonstrating the feasibility of a mathematical model of automobile collisions which could achieve improved uniformity and accuracy in the interpretation of evidence in automobile accidents.
Prior to the creation of SMAC, the general practice in the reconstruction of automobile collisions was to consider the collision and the trajectory phases of the event separately (e.g., [2], [3], [4]). This division of the analytical task was based on two assumptions: (1) that the effects of tire forces are negligible during the existence of collision forces and (2) that the collision event can be assumed to occur instantaneously.
While these assumptions appear to be reasonable, their application had been found to produce significant errors during the collision. For example, in the case of moderate-speed intersection collisions in which multiple contacts frequently occur – front-side followed by side-to-side and or rear-to-side contact ([12]). If secondary contacts are neglected, major errors can be produced in predictions of spin-out trajectories. On the other hand, if tire forces are neglected throughout the time during which the collision contacts occur, significant errors can be introduced in the lateral motions of the vehicle between impacts. Thus, it was deemed essential at the time of the creation of the SMAC program that in a general procedure for reconstruction calculations that both the collision and tire forces be considered simultaneously.
Changes in positions and orientations during the contact phase of collisions can also produce significant changes in the directions and magnitudes of forces and moments acting on the vehicles. Since the early 80’s research [5,6] has revealed that the accuracy of an angular momentum solution procedures for accident reconstruction which includes the assumption of no movement between impact and separation will produce unacceptable error levels (>20%) in many cases.
Other more recent analytical accident reconstruction techniques which are based on conventional momentum analyses include the somewhat subjective input requirement that either a vehicle-to-vehicle contact "point" [7], or a "point of maximum engagement" [8] or an "impact center" [9] be specified. The additional input is required to compensate for the cited solution procedure’s assumption of an instantaneous exchamge of momentum and lack of an independent determination of separation positions and orientations.
The requirement that the user specify either an arbitrary impact contact "point" or an arbitrary "point of maximum engagement" detracts from the objectivity of the reconstruction techniques. Users of the reconstruction techniques, after setting up the vehicles and scene, must decide not only the initial impact configuration but also the point of maximum engagement during each and every collision. This is an undue burden/shortcoming which also permits too much control/leeway on the results of the reconstruction: By an arbitrary choice of initial contact and point of maximum engagement, the analyst can either inadvertently or intentionally bias the results. If you allow 20 engineers to reconstruct a single accident you can get 20 different 'points of maximum engagement' and therefore 20 different results.
SMAC is an "open-form" accident reconstruction program. A requirement of "open-form" programs like SMAC is that the user must initially estimate the impact speeds. The program also generally requires iterations to achieve an acceptable match of the accident evidence.
One of the difficulties which arose in setting up SMAC simulations by the investigative teams was that the initial estimate of the speeds was not always obvious. Also, the user had to provide vehicle properties and specifications, many of which were not readily available. Those requirements, combined with the relatively high cost per run for a SMAC simulation run, required that a pre-processor be created which could provide the initial estimate.
The CRASH computer program [10, 11, 6] was first created to assist SMAC users in determining a first estimate. The original CRASH program utilized both piecewise-linear trajectory solution procedures and a damage analysis procedure to provide an initial estimate. The CRASH program was subsequently adopted by NHTSA as an integral part of the NASS investigations. The rationale for the use of the CRASH program was that for statistical studies, the average error in severity determinations is more important than any individual errors. The CRASH program, with it's question and answer mode, vehicle categorization, single step solution procedure, and most importantly low cost, redirected the NHTSA interest from SMAC towards the CRASH computer program.
The SMAC program was initially developed in the 1970's when all development of computer code was performed on time-share mainframe computer systems. The capabilities of computers at that time were limited by the maximum amount of available memory (e.g., limit on program size) and users were charged for computer use based on memory and CPU utilization. The costs associated with the development and execution of the SMAC program were relatively high (e.g., [12] ,circa 1971,p 48, "The range of costs…has been approximately $25.00 per application run" for the SMAC program). These limitations during the original development of the SMAC program guided the selection of many of the simplifying assumptions of the mathematical model.
Since the early 80's and particularly by the mid 1990’s, the prevalence of powerful mini-computers and more recently extremely powerful and inexpensive Pentium PC’s, creates an availability of virtually unlimited and inexpensive computer resources. This has inspired a detailed re-evaluation and refinement of computer codes, particularly those developed in the 1970's. The general approach to the reported refinements of the SMAC computer program has been to reconsider the initial simplifying assumptions based both on the availability of additional full-scale test results and the virtually unlimited computer resources.
Current Status of SMAC