Crash reconstruction experts have many years of experience and the necessary technical training to evaluate impact speeds using many scientifically validated procedures. In some cases, calculations may be done by hand based on the conservation of linear and/or angular momentum or energy accounting.
However, in most cases, sophisticated software applications are used as these programs allow real time updates of sensitive data adjustments such as impact angles, roadway frictions, and tire/road forces. In all cases no one solution is used; rather, multiple solutions are aimed at finding a reliable and defendable result.
In the end, we want to provide you with answers that will stand the test of rigorous scientific scrutiny.
Often 2 car intersection collisions are resolved by applying the Conservation of Linear Momentum. The Momentum solution determines impact speeds in two vehicle collisions based on the Law of Conservation of Momentum. Some people refer to this as the “360 degree method” of momentum analysis.
The solution uses the separation speeds and simultaneously solves for impact speeds, speed changes, separation speeds at the damage centroids, angles of the principal direction of force, and restitution.
All the equations are found in the instructional literature at Northwestern University’s Traffic Institute, Institute of Police Technology and Management, and other collision reconstruction training facilities around the world. The solution is founded on the fact that linear momentum is conserved, which means the total momentum of the system (i.e. of the two vehicles together) is the same immediately before and immediately after impact. Therefore, if the post-impact momentum can be quantified, then pre-impact momentum is also known and can be used to determine the speeds of the vehicles.
Users of the system must always have well documented evidence confirming the basic set of information that provides the input variables to the solution which include:
1. Vehicle positions at impact.
2. Vehicle orientations at impact.
3. Vehicle positions at rest.
4. Vehicle orientations at rest.
5. Vehicle masses.
6. Vehicle trajectories from impact to rest (i.e. approach and departure angles).
7. Road surface information (i.e. friction, grades, etc.).
The Law of Conservation of Momentum always applies; however, in some situations the results become very sensitive to any inaccuracies in the input values. In these cases, Momentum Genius should only be used with caution and with an understanding of these limitations. These situations include:
1. Situations where the above information set is incomplete (i.e. if the separation speeds or angles are estimated, the accuracy of the results will be affected by the accuracy of those estimations).
2. Collisions of a more head-on nature where the angle is less than about twenty degrees (in these cases the results will be very sensitive to inaccuracies in the approach and departure angle for both vehicles).
3. Collisions where the mass disparity of the vehicles exceeds four to one (in these cases the results will be very sensitive to inaccuracies in the angles for both vehicles and to inaccuracies in the weight and separation speed for the heavier vehicle).
4. Sideswipe collisions where vehicles involved do not completely engage (the Momentum Genius will still work but you will get an error message that the common velocity assumption was violated. That is expected since in a sideswipe the vehicles are moving past one another even during impact, and therefore no common velocity is achieved. You can safely ignore this error message in this case.
The accuracy of the linear momentum solution is well known and documented by other specific linear momentum solutions via the ricsac crash tests. *
The Momentum Genius solution has been validated by applying it to a specific case where the evidence is well documented from a staged crash. A crash test performed by Exponent was used to verify the application of this tool and provided results within 2% of the actual speeds.