Computerized Reconstruction of Accident Speeds on the Highway, EXtended uses conservation of momentum and energy to find initial speeds from initial headings and trajectories to rest. It is based on the U S Department of Transportation's classic CRASH program of the 1970's, but with significant improvements in accuracy and versatility. For purposes of comparison CRASHEX reverts on command to the original assumptions of CRASH, yielding an alternate set of outputs based on identical inputs. While the speed of each of two vehicles prior to impact is readily found - - even graphically - - by conservation of momentum, given the speed of each upon separation and all four directions, those speeds at separation are difficult to evaluate for a vehicle which thereafter travels in part more or less sideways, or "spins," with fluctuating deceleration. The problem is to assign it a rate of deceleration with reasonable accuracy. Rather than to simply ignore this problem or rely on experience-based user estimates, CRASH has a built-in base of experience in the form of mathematical curve fits to a family of mathematically modeled spins. In comparison with any momentum-based treatment which lacks such built-in routines, CRASH (in all its embodiments) offers significantly better accuracy of reconstruction in all but the simplest cases of travel to rest. In part due to its superior treatment of spin, the historic CRASH treatment, available from various sources, is at least in the U S a mainstay of accident reconstruction. In some or indeed many instances the unimproved treatment is “good enough,” so that as a basis for informed opinion either treatment serves the same purpose. But at least a final check of CRASH against CRASHEX is needed to be sure that the more exact values do not alter alter the end opinion. CRASHEX is also needed whenever the effects of mis-measurement on the reliability of the CRASH (or other) result will be brought into question - - which will become increasingly likely as awareness of this concept in general, and Daubert challenges in particular, become more common. While both CRASH and CRASHEX find the speeds of approach of two moving vehicles to impact (the V0's) and the speed changes (the delta-V's) during impact both by momentum (given site data) and by energy (given damage data), CRASHEX alone recites the delta-V redundancy, the shear ratio, and the coefficient of restitution -- as quality ratings which tell you whether the solution is internally consistent and physically valid. includes the effects of tire forces acting during impact. These forces are included in CRASHEX just as they are in SMAC; and indeed by use of the same model of tire behavior. While such differences may be small in instances of large travel to rest after impact, tire force effects otherwise could swing the balance of useful opinion. The expert might best then select CRASHEX as the preferred means of reconstruction. solves the case of obstacle impact. This is so whether the obstacle is a stationary vehicle or an immovable barrier.

The sole computerized alternative to time-reversed momentum-and-energy-based treatment is a simulation, a time-forward mathematical re-enactment using simple kinematics and good models of tire response to operating conditions, such as that provided by SMAC, from the same DOT source. This approach requires for its initiation a hopefully good guess as to its initial state, or (as was the original intent) a prior run of CRASH or the like. If in good agreement they have confirmed each other. Due to the robustness of the kinematic and tire models, such agreement is probably more significant than agreement with error-prone observation of staged collisions. CRASHEX in any event goes on to add a statistical treatment of deviations of measurement not otherwise offered regardless of treatment. Given a carefully chosen "best" solution, the user can enter (from a look-up) the 5% likely (2 SD) error of measurement of every input and find each individual effect on each output and the likely total effect of all these component uncertainties. The result is the less than 1-in-40 likely high and likely low extremes of your solution. The expert is then prepared to answer with confidence all reasonable What-If questions about his or her opinion. The utility of CRASHEX lies firstly in its more-accurate treatment of both impact and post-impact travel, secondly in the insight gained through a greater number of associated outputs, and thirdly in the provision of Finite Difference Analysis for evaluation of the statistical span of values about the mean approach speeds. Furthermore, that numerical span is directly transferable from CRASHEX to CRASH or SMAC or any other preferred basis of testimony, because the bell curve expressing the distribution of values of each approach speed is a function of the actual event and its investigation, not of the method of reconstruction. Its span thus must be the same for all means of reconstruction, even if it is centered on a somewhat different mean. The same bell curve applies even to a simulation, which practically speaking cannot be solved for that result.

CRASHEX executes so rapidly that the program's total run time of consists almost entirely of the user's input time at the keyboard plus the printer's output printing time. CRASHEX thus can in a few hours of use add the “final polish” to a reconstruction already developed over days and months of investigation and analysis. This is why it is available on (and only on) a short-term, low-cost, essentially time-share basis. The usage fee can be charged to your client as an item of expense, no more per day than you charge for an hour of your services. You list it for later reimbursement when you bill for your services. NEXT