Kwasnoski’s Quick Tips

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Professor Kwasnoski has teamed with The Traffic Safety Guy to share tips with prosecutors and law enforcement officers for effective crash investigation and prosecution.

Click on the heading below to read the full tip.


In a pedestrian criminal case a defense reconstruction expert opines that the available sight distance for the defendant was less than the stopping distance required, and therefore the collision was unavoidable. The expert’s report includes the following language:   “The police report of this incident includes a determination that the defendant’s vehicle was approximately 220 ft from the POI when the defendant operator first perceived the pedestrian’s presence in the roadway. Using a generally-accepted perception-rection time, the police reconstructionist opines that “…this collision is avoidable at the posted speed”. This statement does not accurately reflect the accepted stopping distance for the posted speed published in the state’s own driver’s education manual. The New Hampshire Driver’s Manual, 2001 edition, p. 44 states that the total stopping distance for a vehicle traveling at a speed of 50 mph is 243 ft. Thus, the defendant’s vehicle, if operating at 50 mph would not have been able to stop in the distance of 220 ft determined by police to be the point of first perception of the pedestrian by the defendant. Clearly, this accident was unavoidable, even at the posted speed of 50 mph, because it was the careless entry into the roadway by the pedestrian that resulted in her own death.”


Comment: The published chart is meant to give new drivers a sense of the safe following distance, and is developed to include driving under all conditions, including rainy weather in which the road surface would be less frictional than when dry. This would be reflected in the portion of the chart labeled “Braking Distance”, and in this particular chart the road friction, or drag factor, used in calculating the values in the chart can be determined to be only 0.44.

A drag factor of f = .44 would be consistent with a wet road surface, and this chart uses this low value to allow for safe following distance even in inclement weather. In the case in question, however, the road surface was dry and a drag factor of .82 was measured by police investigators. Using the dry road friction value, the braking distance would be calculated to be only 101 ft; the stopping distance would be 55 + 101 = 156 ft. For the dry road surface the defendant, having 220 ft of available stopping distance, could stop before striking the pedestrian if he had reacted in time.

This is a case where a published chart may be used to confuse the issue of negligence if it is accepted without regard to its applicability to the particular case. When using a chart or design document of any kind the user must know what the parameters are that were used to construct the chart, and thus, whether the chart even applies to the specific case. In general, highway design charts are usually constructed for roads under the worst weather conditions since the purpose of the chart is to afford safety to operators under all conditions. Beware of the civil engineer or highway design engineer who employs design charts or highway guidelines as a basis for an opinion about the culpability of an operator; the misuse or misunderstanding of design charts may lead the expert to opine about a different roadway condition than that of your case.

Crash Investigators

Crash investigators should check the NHTSA website ( for recalls and technical service bulletins (TSB’s) , and for complaints to check that there is no legitimate mechanical failure or defect that could have been causative. And don’t forget that if there is a recall that may be causative you should check to see whether the recall was satisfied. The NHTSA complaints file is often overlooked in this search, but could reveal information very relevant to a particular crash.

A recent case that I reviewed involved a 2002 Ford vehicle that suddenly swerved, causing serious personal injury. One of the photographs of the defendant’s vehicle showed the right front wheel toed in severely, with no evidence of any impact damage to have caused it. In a quick search of the NHTSA site I found more than 500 complaints on this vehicle model, and in reading through file there were numerous complaints of the right front suspension failing due to excessive corrosion of the right front of the vehicle. Since the crash was not a fatality, the police had not done a mechanical inspection as part of their investigation, and they had not checked for recalls or other reported problems. The complaints file alerted me to the possibility that a mechanical failure had caused the crash.

Make a check for recalls, TSB’s, and complaints a routine part of a MV crash investigation protocol. I may reveal potentially exculpatory evidence, and increases the credibility of the investigation.

Defense council elicits from the Law Enforcement witness that the defendant may have applied the brakes during the yawing motion; a challenge questions the validity of the CSY equation if the operator applies braking during the yaw based on quotes from published treatises. [1][2]

Rivers: p. 342:  “Do not use formula C6-18 (CSY) if …the brakes were applied.”

Fricke: p. 72-35: “The equations … are derived with the assumption that…neither braking nor power was applied to the vehicle during the yaw.”

The Rivers quote is incorrect, and the Northwestern text by Fricke in a paragraph just a few pages later (p. 72-42) states that: “tests also indicate that substantial braking, short of locking the tires, does not affect the velocity estimates significantly.”

Trooper David Dye of the New York State Police, published results of his testing with ABS and non-ABS vehicles [3] showing that application of brakes during a CSY event did not affect the validity of CSY calculations of speed.  That reference states: “Braking during the yaw, short of locked-wheel braking, does not affect the application of the CSY equation.” Numerous other such testing can be found in the literature. [4]


[1] R. W. Rivers, Training and Reference Manual for Traffic Crash Investigation, 3rd Ed, IPTM, 2008.

[2] Fricke, Traffic Accident Reconstruction, Vol. 2, 1990.

[3] Dye, “Application of Drag Coefficients to ABS Related Sideslip”, Collision, February,2007.

[4] Kwasnoski, Kwasnoski’s Little Red Book,, p. 104.

A defense expert cites a published paper [1] that claims that the CSY equation is invalid. This paper includes the statement, “The critical speed formula calculations were greater than the measured velocities of the test vehicle in all test results.” This would be a troubling assertion, and could be used to attack the CSY calculation in a motion to exclude. The published research, however, may have been done to show that two maneuvers which are not CSY maneuvers – a right circle turn (the vehicle is put into a sharp radius steering maneuver and then the vehicle speed is increased until the tires lose traction), and a double-step steer maneuver (which is a quick, jerky, swaying motion consistent with a sudden driver avoidance or lane change maneuver) should not be analyzed using a CSY calculation.

Since neither of these should be analyzed by using the CSY formula the testing shows that misapplication of the CSY would produce unacceptable results. The expert portrays to the Court, however, that this publication means that the CSY equation itself is invalid. When an expert makes a claim based on a published treatise, and if the discovery in the case facilitates, a copy of the paper must be obtained so you own expert can review it and be prepared for rebuttal or to help construct a destructive cross examination. Studies validating the CSY equation are included in Kwasnoski’s Little Red Book.

[1] Dickerson, “Evaluation of Velocity Predictions Using the Critical Speed Formula”, SAE# 950137

In a recent case, MA v. Cruz, the defense motion to exclude State Police reconstruction testimony included this language:

“Yaw can be considered definite at the point where the rear tire path crosses over the front tire path. (Rivers, Training and Reference Manual for Traffic Accident Investigation, IPTM 1995, p 390) Furthermore the literature provides that measurements used to determine the radius of a critical speed yaw should begin at the point at which the rear tire mark intersects with the front tire mark in a critical speed yaw. Here, where no such point of intersection [crossover] exists, the radius could not have been determined consistently with the methodology called for by the critical speed formula.”

Ruling of the Court: “For these reasons, the Trooper’s opinion based is based on an “insufficient evidentiary foundation”, and is subject to exclusion. The defendant’s Motion to Exclude is granted.

The assertion “Furthermore the literature…” is false, and is a misinterpretation of the statement, “yaw mark measurements should be made early in the yaw, as close to the crossover point as possible.” This does not say that a crossover point must be visible, but simply that measurements should be made where the first evidence of the rear tire being outside the front tire is seen (Daily, Fundamental of Traffic Crash Reconstruction, IPTM, 2006). John Daily email to this author, 3/29/13 “As a matter of clarity, they do not have to see the actual place where the out tracking begins (crossover), just that it has begun (rear tire tracks outside front tire).”

An operator of a motor vehicle traveling in the second of four Northbound lanes, with moderate traffic in the other lanes, encounters a dangerous situation in the roadway. The operator looks in his side mirror and assesses the availability of a lane change to his left, but determines that there is no available space in traffic in that lane, and aborts the lane change consideration (he clearly describes this attempted lane change in his statement to police). Now, after traveling an additional distance while assessing the possibility of this lane change, his vehicle is too close to the danger to brake successfully to a stop, and a collision occurs.

A police reconstructionist evaluates the point of first possible perception of the danger, and in a re-creation of the scene is able to determine an available distance to avoid the collision. At the posted speed of 55 mph (determined by reconstruction of the crash), it is determined that the driver had 4.95 seconds to initiate an evasive action. If the operator has applied emergency braking with a PRT of 1.5 seconds the reconstructionist calculates the total stopping distance to be only 255 ft; the reconstructionist’s opinion is that this collision was avoidable (if the operator had initiated full braking as his first response to the danger).

The human factors literature discusses the time it takes to make the decision to change lanes (or not). “Finnegan and Green (1) reviewed five articles concerned with driver lane-changing behavior. The data indicate that, typically, drivers will make between 2 and 3 head movements (checking mirrors or looking to the side), which consume 1.0 to 1.5 seconds each. When glances to the road ahead are included the findings are that preparatory visual search would range from 3.7 seconds in the absence of traffic , up to 6.6 seconds when traffic is present.” Using this range at 55 mph this would use up a distance of at least 299 ft, leaving only 101 ft to execute a different evasive action, which would not be enough to successfully brake to a stop.

Another reference (2) states, “It is appropriate to consider decision sight distance as a criterion any time a roadway situation requires a driver to carry out a relatively complex maneuver such as a lane change in traffic.” “The basic work to develop the guidelines for decision sight distance was carried out by McGee, et al (1978).” The McGee range of time to make the lane change decision is 4.2 to 7.0 sec.  Using this range at 55 mph this would use up a distance of at least 339 ft, leaving only 61 ft to execute a different evasive action, which would not be enough to successfully brake to a stop.

This case reminds us that the correct application of the law may not be as straightforward as comparing some numbers or doing a reconstruction calculation. A charging decision would include the question, “Was this criminal negligence, or just the wrong choice when the operator first considered avoiding the collision by changing lanes?”

(1)  Finnegan and Green, “The Time to Change Lanes: A Literature Review”, UMTRI report # UMTRI-    90-34, p. 184

(2)  McGee et al, “Decision Sight Distance for Highway Design and Traffic Control Requirements”,  U.S. DOT, FHA report No FHWA-RD-78-78, p. 184

Dragging a sled to determine friction
Dragging a sled to determine the friction.

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About John Kwasnoski

John B. Kwasnoski is Professor Emeritus of Forensic Physics at Western New England University, Springfield, MA, after thirty-one years on the faculty. He is a certified police trainer in more than twenty states, and has instructed prosecutors, police, and civil attorneys on more than 250 occasions across the U.S. He is the crash reconstructionist on the “Lethal Weapon - DWI Homicide” team formed by the National Traffic Law Center to teach prosecutors how to utilize expert witness testimony and cross examine adverse expert witnesses.

Prof. Kwasnoski has reconstructed more than 1,200 crashes, including multiple and single vehicle, pedestrian, motorcycle, and train crashes, and has given sworn testimony on more than 200 occasions; he has been training the NYPD collision reconstruction unit since 2001; and continues to serve as a consultant to prosecutors nationwide on MV homicide cases. He has worked for more than twenty major insurers as a consultant/expert witness, and has conducted training for claims adjusters and special investigators for a number of insurance companies.

Prof. Kwasnoski is the author and co-author of several books, has published more than 60 journal and newsletter articles on collision reconstruction, and maintains an active speaking schedule nationwide.

Legal Sciences

Legal Sciences , John Kwasnoski's website, is dedicated to providing information, educational resources and training opportunities for professionals involved in the criminal justice system who handle impaired driver and motor vehicle crash cases.