- Brian G. McHenry, McHenry Software, Inc., Cary, NC
©McHenry Software, Inc.
Presented at the SAE Accident Reconstruction state-of-the-art TOPTEC ® December 10, 1999
This article also appears on the McHenry Software website
- The Simulation Model of Automobile Collisions (SMAC) computer program, developed in the early 1970's, includes a complex collision algorithm for monitoring, detecting and modeling the collision interactions of motor vehicles. A detailed review of the logic, rationale and limitations of the original SMAC collision algorithm as well as extensions and refinements which have been implemented in the various versions of the SMAC computer program will be discussed.
- The acronym SMAC stands for the Simulation Model of Automobile Collisions. The computer program is a time-forward simulation model. With SMAC you set up a mathematical "full scale crash test" using the vehicle weights, dimensions and other properties, you set the initial positions, headings and velocities and start the simulation run. The vehicles collide and run out to rest. You then compare the SMAC predicted positions, orientations at rest and predicted damage with the accident evidence.
SMAC was initially developed for government sponsored research in highway safety. It is currently being used mainly with respect to litigation. Whatever the use of SMAC, or its equivalent, limitations on accuracy and generality that were imposed by early 1970’s vintage mainframe computers should not persist.
But first id like to present a little background on the SMAC computer program:
- 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 . 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 nations 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., , , ). 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 (). 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" , or a "point of maximum engagement"  or an "impact center"  be specified. The additional input is required to compensate for the cited solution procedure’s assumption of an instantaneous exchange 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.,  ,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.
- In the early 1970’s, NHTSA sponsored a research project to develop a computer program that would achieve improved uniformity, as well as improvements in accuracy and detail, in the interpretation of physical evidence in highway accidents. The resulting prototype computer program was the Simulation Model of Automobile Collisions (SMAC) [12-15]. At the completion of the NHTSA sponsored research at Calspan in 1974, a preliminary version of the SMAC program was delivered to the NHTSA and it has subsequently been distributed as the NHTSA SMAC computer program.
Subsequent follow-up contracts for research and development of the SMAC program sponsored by NHTSA went to other organizations [16-18]. Further research and development on the SMAC program was also continued independently at Calspan  and additional corporate-sponsored research to support criticism of the SMAC program [20,21] was also performed. There were no significant changes by NHTSA into the 1974 NHTSA SMAC at the completion of the NHTSA follow-up contracts.
In 1986, Day and Hargens created EDSMAC , a PC version of the 1974 NHTSA SMAC program converted to the BASIC programming language. Subsequent reports related to the EDSMAC program [23-25] reveal that except for very minor modifications, the EDSMAC program is essentially the same as the original 1974 NHTSA SMAC program. Related development efforts by the distributors of the EDSMAC program have been directed towards a mini-computer based high-end graphics environment [25-29].
In 1988, a number of suggestions for further refinement and extensions of the SMAC program were presented . In 1989, some suggestions for avoiding misapplication of computer programs, including the EDSMAC program  were presented.
In 1997, further suggestions for extension and refinement of the SMAC computer program were presented  and suggestions for refinement of the restitution portion of the collision model of SMAC were presented .
In 1999, EDSMAC4  claims to include a "more realistic modeling of actual vehicle structural behavior" based on "an A, B stiffness model". Please note that the "A,B stiffness model" is a "virtual" model equating residual crush to dissipated kinetic energy. The "A,B stiffness model" was not intended to be a dynamic model. With the "A,B stiffness model" a very stiff near plastic vehicle can share the same "A,B stiffness" as a very soft near elastic vehicle. The residual crush on a vehicle does not tell you anything about the vehicle stiffness EXCEPT in the case of a plastic vehicle. Motor vehicles crushing in collisions are not plastic. They have restitution.
Restitution consists of two separate aspects: (1) a partial dimensional recovery and (2) a partial restoration of kinetic energy. In our 1997 SAE paper on restitution , we pointed out that the current implementation of collision modeling in the SMAC computer program includes identical loading and unloading load-deflection rates and that the unloading (restitution) phase of the collision was at near peak levels. The effect of partial restoration of the energy at near peak levels is that the SMAC program acts to return part of the absorbed energy but it does so with a less-than-actual dimensional recovery.
EDSMAC4 also has included in the "A,B stiffness model" a "threshold force to be applied before deformation begins". What mechanism exists in the real world which provides a force without a deflection? Are they assuming that the bumper is a pre-loaded spring? You have to do work to absorb energy in the vehicle structure. Work equals Force * Displacement. The effect of the changes in EDSMAC4 is that the acceleration peaks are higher and the duration of the impact impulse is shorter.
EDSMAC4 has chosen to artificially stiffen the vehicle to better match the residual crush. In effect they have created a "residual crush" stiffness model which reduces the validity of the modeling of the loading phase of the collision. The most important aspect of a collision is the loading phase.
The current PC versions of the SMAC program available commercially are EDSMAC, WinSMAC  and m-smac . These programs are currently being utilized in individual case reconstructions. This presentation will include a detailed discussion of the SMAC collision model and a summary and discussion of the some of the related extensions, modification and refinements from the cited reports.
- 1. "A Review of ACIR Findings", Campbell, B.J., 8th Stapp Car Conference, 1966
2. "On the Mechanics of Vehicle Collisions", E. Marquard, Automobiltechnische Zeitschrift, Vol. 64, No 5 (May 1962) pp. 142-148
3. "Progress in the Calculations of Vehicle Collisions", E. Marquard, Automobiltechnische Zeitschrift, Vol. 68, No. 3 (1966), pp. 74-80
4. "Non-Central Collisions of Two Rubber Tired Vehicle", G. Bohm and E. Horz, Automobiltechnische Zeitschrift, Vol. 70, (1968), No. 11, pp. 385-389 and No. 12, pp. 428-432
5. "National Crash Severity Study - Quality Control, Task V: Analysis to Refine Spinout Aspects of CRASH", McHenry, R.R., McHenry, B.G., , Calspan Field Services, Inc. ZP-6003-V-4; DOT-HS-6-01442, January 1981
6. "CRASH97-Refinement of the Trajectory Algorithm", SAE Paper 970949
7. "Linear and Rotational Momentum for Computing Impact Speeds in Two-Car Collisions (LARM)" ,Limpert, R., Andrews, D.F., SAE paper 91-0123
8. "The Collision and Trajectory Models of PC-CRASH", Steffan, H., Moser, A., SAE Paper 96-0886
9. "Impact Center and Restitution Coefficients for Accident Reconstruction", Ishikawa, H., Japan Automobile Research Institute, SAE Paper 94-0564
10. "The CRASH Program – A Simplified Collision Reconstruction Program", McHenry, R.R., Proceedings of the Motor Vehicle Collision Investigation Symposium, Calspan, 1975
11. "User’s Guide for the CRASH Computer Program", McHenry, R.R., Lynch, J.P., Calspan report No. ZQ-5708-V-3, Contract DOT-HS-5-01124, Jan 1976
12. "Development of a Computer Program to Aid the Investigation of Highway Accidents", McHenry, R.R., Contract FH-11-7526, NTIS PB 208537, Calspan Report VJ-2979-V-1, December 1971
13. "A Computer Program for Reconstruction of Highway Accidents", McHenry, R.R., SAE Paper 73-0980, Proceedings of the 17th Stapp Car Conference, Nov 1973
14. "Mathematical Reconstruction of Highway Accidents", McHenry, R.R., Segal, D.J.,Lynch, J.P., Henderson, P.M., Contract DOT-HS-053-1-146,NTIS PB 220150, Calspan Report ZM-5096-V-1, January 1973
15. "Mathematical Reconstruction of Highway Accidents - Scene Measurement and Data Processing System", McHenry, R. R., Jones, I. S., Lynch, J. P., Calspan Corporation, Contract DOT-HS-053-3-658, Calspan Report ZQ-5341-V-2, December 1974
16. "Revision of Simulation Model of Automobile Collisions (SMAC) Computer Program: Investigation of New Integration Algorithm", Chi, M., Neal, E., Tucker, JR, Contract DOT-HS-7-01545, Report No. DOT-HS-803 294
17. "Improvement of Accident Simulation Model and Improvement of Narrow Object Accident Reconstruction", James, M.E., Ross, H.E., Whittington, C., April 1978, Contract DOT-HS-5-01262, DOT-HS-7-1656, Report DOT-HS-803 620
18. "A Computer Model to Operate the SMAC Program Automatically", Moffatt, C.A., Byrd, J., 1980, Contract DOT-HS-8-01820, in Highway Collision Reconstruction, ASME winter meeting, 1980, Library of Congress 80-69198
19. "Computer Aids for Accident Investigation", McHenry, R.R., SAE Paper 76-0776
20. "The Accuracy and Usefulness of SMAC", Warner, C.Y., Perl, T.R., SAE Paper 78-0902
21. "Improvements to the SMAC Program", Perl, T.R., Anderson, D.O., Warner, C.Y., SAE Paper 83-0610
22. "Vehicle Analysis Package - EDSMAC Program Manual, Version 2", Engineering Dynamics Corporation, Lake Oswego, OR 1986
23. "An Overview of the Way EDSMAC Computer Delta-V’, Day, T.D., Hargens, R.L., SAE Paper 88-0069
24. "Further Validation of EDSMAC using the RICSAC Staged Collisions", Day, T.D., Hargens, R.L., SAE Paper 90-0102
25. "Validation of Several Reconstruction and Simulation Models in the HVE Scientific Visualization Environment", Day, T.D., Siddall, D. E., SAE Paper 96-0891
26. "The Scientific Visualization of Motor Vehicle Accidents", Day, T.D.,SAE 94-0922
27. "An Overview of the HVE Developer’s Toolkit", Day, T.D., SAE Paper 940923
28. "An Overview of the HVE Vehicle Model", Day, T.D., SAE Paper 95-0308
29. "An Overview of the HVE Human Model", Day, T.D., SAE Paper 95-0659
30. "SMAC-87", McHenry, B.G., McHenry, R. R., SAE Paper 88-0227
31. "Application and Misapplication of Computer Programs for Accident Reconstruction", Day, T.D., Hargens, R.L., SAE Paper 89-0738
32. "SMAC97- Refinement of the Collision Algorithm", McHenry, B.G., McHenry, R.R., SAE paper 97-0947
33. "Effects of Restitution in the Application of Crush Coefficients", McHenry, R.R., McHenry, B.G., SAE Paper 97-0960
35. "An Overview of the EDSMAC4 Collision Simulation Model", Day, T., SAE paper 1999-01-0102