Speed Kills—Or Does It?

Few road-safety slogans have achieved the global reach of “Speed Kills.” In South Africa, it appears on roadside billboards and enforcement campaigns. In the United States, it underpins federal and state initiatives from the National Highway Traffic Safety Administration (NHTSA) to state patrol saturation operations. Around braais, bars, and boardrooms alike, everyone has an opinion on speed and speeding.

But what does a collision reconstruction specialist—someone who measures crush, calculates forces, and testifies under oath—actually say?

Stan Bezuidenhout, forensic road traffic collision reconstructionist and court-recognized expert, has attended thousands of crash scenes across jurisdictions. His work involves the scientific analysis of evidence: vehicle deformation, roadway marks, energy transfer, momentum exchange, and occupant kinematics. His conclusions are tested in court—whether under South African evidentiary standards or, in the U.S., under the rigours of Daubert admissibility scrutiny and Federal Rule of Evidence 702.

When asked whether “speed kills,” his response is disarmingly direct:

“Speed, in isolation, kills nothing. Speed is simply distance divided by time. The slogan is effective marketing—but scientifically incomplete.”

That statement unsettles people. It should.

The Marketing of a Villain

From a communications perspective, “Speed Kills” is brilliant. It is short, emotive, and easy to enforce. If speed is the villain, enforcement becomes the hero. Revenue generation through fines becomes defensible policy. Public buy-in follows repetition.

In both South Africa—under the National Road Traffic Act 93 of 1996—and in the United States, speed limits are statutory requirements. Exceeding them constitutes an offence. No debate there.

But the scientific question is different:
Does speed alone cause crashes—or does it merely amplify consequences once other failures occur?

Bezuidenhout draws a distinction between legal culpability and physical causation. Courts assess negligence, statutory breaches, and foreseeability. Physics assesses motion, energy, and force. The two overlap—but they are not identical.

Speed Versus Delta-V: What Actually Injures People?

In reconstruction science, severity is not measured by posted speed limits. It is measured by change in velocity, commonly referred to as Delta-V.

Delta-V is the amount by which a vehicle’s velocity changes during a collision event. Injury risk correlates far more directly with Delta-V than with pre-impact speed in isolation.

A vehicle travelling at 120 km/h that continues in the same direction, without incident, injures no one.
A vehicle travelling at 60 km/h that strikes a rigid barrier and comes to an abrupt stop experiences a substantial Delta-V—and occupants may suffer severe trauma.

The governing principle is not “speed kills,” but rather:

The greater the change in velocity over a shorter time interval, the greater the forces imparted to occupants.

That is Newtonian mechanics. Not opinion.

In U.S. litigation, experts routinely reference SAE (Society of Automotive Engineers) literature and NHTSA injury criteria linking Delta-V to occupant risk thresholds. In South African courts, similar biomechanical principles apply, even if expressed less formally.

The distinction matters.

Traffic Friction: The Missing Variable

Speed rarely exists in isolation. Real-world roads are dynamic systems. Bezuidenhout uses the term traffic friction to describe disruptions in smooth traffic flow.

Examples include:

• Driver distraction (cell phone use)
• Inconsistent lane discipline
• Heavy vehicles interacting with light motor vehicles
• Variable driver risk tolerance
• Sudden braking in response to enforcement presence
• Poor infrastructure design or signage

When traffic flows uniformly—such as on a racetrack with drivers moving in the same direction at similar speeds—incidents are statistically rare despite high velocity.

When traffic friction increases, conflict points multiply. It is the differential in speed, the abrupt lane change, the unexpected stop, or the loss of control that creates the collision—not velocity in a vacuum.

This distinction is critical for attorneys assessing negligence. Was the breach excessive speed per se—or was it failure to maintain proper lookout, following distance, lane discipline, or vehicle control? The legal framing often defaults to speed because it is easy to measure and easy to prosecute.

Legal Responsibility Versus Physical Reality

Courts in both South Africa and the United States do not require speed to be the sole cause of a collision for liability to attach. Under negligence principles, excessive speed relative to conditions may constitute breach of duty—even if other factors contribute.

In U.S. jurisprudence, comparative negligence frameworks allow apportionment where speed is one factor among many. In South Africa, contributory negligence principles similarly recognize shared causation.

But that is not the same as saying “speed kills.”

Speed may:

• Reduce reaction time
• Increase stopping distance
• Amplify kinetic energy
• Increase crash severity

However, it does not independently initiate a collision absent other failures—driver error, mechanical defect, roadway design deficiency, or environmental conditions.

Enforcement, Revenue, and Public Policy

Speed enforcement has measurable deterrent value in certain contexts—particularly where extreme speed variance exists. No credible reconstructionist advocates for unrestricted velocity on public roads.

But oversimplification carries risk. When campaigns focus exclusively on speed, other contributors—fatigue, impairment, distraction, maintenance failures, infrastructure neglect—receive less attention.

For attorneys handling civil or criminal collision matters, this has practical implications:

• Was the reconstruction based on comprehensive dynamics analysis or a single-variable speed assumption?
• Was Delta-V assessed?
• Were braking phases, surface transitions, and energy absorption properly evaluated?
• Was traffic friction considered?

Overreliance on speed can obscure more probative factors.

What Actually Reduces Harm?

Evidence consistently shows that injury mitigation improves when:

• Vehicles incorporate advanced safety systems
• Infrastructure separates opposing flows
• Traffic patterns reduce speed differentials
• Enforcement targets high-risk behaviours (e.g., impairment, reckless overtaking)
• Driver training emphasizes hazard perception and gap management

Speed management plays a role—but only within a broader systems approach.

A More Accurate Statement

If precision mattered more than marketing, the slogan might read:

Greater changes in velocity over shorter time intervals increase the likelihood and severity of injury.

It lacks billboard appeal—but it reflects physics.

The Strategic Perspective

For practitioners—whether prosecutors, defense attorneys, insurers, or risk managers—the key takeaway is this:

Speed is a factor. It is not a universal explanation.

When analysing or litigating collision cases in South Africa or the United States, separating legal violation from physical causation is essential. Courts demand causation evidence—not slogans.

Speed does not kill in isolation.
But unmanaged energy, abrupt deceleration, and systemic failures do.

Understanding the difference is not philosophical—it is evidentiary.