What Attorneys Should Know About Impact Mechanics in Rear-End Collisions by Zach Marshall
Rear-end collisions look simple on paper — one vehicle hits the back of another, liability seems obvious, and the injury mechanism appears straightforward.
But when I review these cases for attorneys, the mechanics often tell a much more complex story.
Understanding how energy moves through both vehicles, how the occupant’s body reacts, and how timing affects injury potential can dramatically strengthen both DUI and PI case strategy.
This is what attorneys need to know.
1. Rear-End Collisions Aren’t Purely “Front-to-Back” Impacts
Most people imagine a clean, straight-line push forward, but rear-end collisions frequently involve:
• Angular contact
• Offset bumper engagement
• Vehicle rotation
• Lane drift
• Secondary impacts
Even slight variations in angle or overlap can change the energy transfer — and therefore the injury pattern — by a significant margin.
This is where insurers often oversimplify, and where attorneys gain leverage by challenging those assumptions.
2. Low Visible Damage ≠ Low Injury Potential
One of the most common myths in PI cases is that minimal vehicle damage means minimal injury.
Mechanically, that’s not how rear-end collisions work.
Many modern vehicles are designed to absorb energy without showing dramatic deformation.
This can result in:
• Significant occupant acceleration
• High peak forces to the neck and spine
• Seatback ramping injuries
• Delayed-onset symptoms
I regularly see cases where the “simple bump” caused very real trauma — and the physics back it up.
3. The Occupant’s Body Doesn’t Move in a Single Motion
Rear-end impacts create a sequence of forces that occur within fractions of a second.
The body experiences:
Initial torso acceleration
Neck hyperextension
Neck rebound into flexion
Forward momentum into restraints
This “S-curve” motion is the mechanical basis for many whiplash injuries.
Understanding it helps attorneys counter insurer claims that symptoms are exaggerated or unrelated.
4. Head Position and Awareness Change the Injury Outcome
Whether the occupant was:
• Looking forward,
• Turned sideways,
• Looking down at their phone, or
• Leaning toward the center console
…directly affects the injury mechanism.
An unaware or rotated occupant generally has higher injury susceptibility because the muscles don’t brace and the neck isn’t aligned with the force path.
This detail often doesn’t appear in police reports — but it’s critical.
5. Brake Application Matters More Than Most People Realize
If the struck driver was braking at the moment of impact, it changes:
• Weight distribution
• Seatback engagement
• How far the body travels
• Whether a secondary impact occurred
This also affects whether the collision was avoidable.
Brake lamp timing can make the difference between 0% liability and partial fault for the rear driver.
6. Pre-Existing Conditions Are Not a Liability-Weakness — They Often Increase Injury Risk
From a biomechanical standpoint, a person with:
• Prior neck injury
• Degenerative disc disease
• Previous back surgery
…is actually more vulnerable to new trauma.
Independent review helps attorneys explain why the collision exacerbated those conditions rather than being unrelated.
Insurers frequently try to spin this the opposite way.
Physics says otherwise.
7. DUI Rear-End Collisions Require an Even Closer Look
When impairment allegations are in play, the prosecution often assumes:
“DUI = caused the crash.”
But rear-end collisions frequently involve:
• Traffic slowdowns
• Unexpected stops
• Visibility issues
• Multi-vehicle dynamics
The core question is not “Was the driver impaired?”
It’s “Did impairment contribute to the crash sequence?”
A technical review often reveals the answer is no — and that another driver or environmental factor initiated the sequence.
8. Why Attorneys Bring Me Into Rear-End Cases
Rear-end collisions generate more assumptions than almost any other crash type.
An independent analysis gives attorneys:
• A defensible breakdown of impact mechanics
• A clear injury mechanism
• Insight into braking, speed, and timing
• A stronger negotiation position
• A physics-based explanation to counter insurer or prosecution claims
Rear-end crashes may be common — but the way they’re analyzed can make or break a case.
The Bottom Line
Understanding impact mechanics transforms rear-end collisions from “routine” to strategically powerful cases.
My role is to give attorneys a clean, defensible, physics-based explanation that elevates both liability arguments and injury support.
Contact Zachary Marshall for more details.
