The physics are clarifying.

What Newton’s Laws Tell Us About Road Speed

Using Newton’s laws of motion, we can calculate the collision speed of an object in free fall from a given height. This gives us a useful comparison point, because most people have an intuitive and healthy fear of falling from height that they do not apply to driving.

A free fall from 10 feet, the height of a single-storey building, produces a collision speed with the ground of approximately 17 mph. A fall from 20 feet, two storeys, produces approximately 24 mph. A fall from 30 feet, three storeys, produces approximately 30 mph. This is roughly the speed of driving through a residential neighbourhood to drop children at school.

A fall from 120 feet, equivalent to a 12-storey building, produces a collision speed of approximately 60 mph. This is a standard North American highway speed.

The weight of the object does not change these numbers. Whether the falling object weighs 10 pounds or 2,000 pounds, the terminal velocity at impact is the same. Mass affects force but not the velocity calculation from free fall height.

What this means practically: a vehicle collision at 60 mph subjects the body to deceleration forces equivalent to falling from the twelfth floor of a building. Most people would not describe falling from a twelve-storey building as a minor event. Most people would not expect no injury from that fall. Yet the same forces, delivered through a car collision at highway speed, are routinely described as minor, and the injuries that follow are routinely undertreated.

Why We Misjudge the Risk

There are several reasons why drivers and passengers consistently underestimate the forces involved in road travel, and understanding these reasons matters for how we approach injury assessment after accidents.

The first is a perceptual phenomenon called velocitization. When a driver or passenger maintains a consistent speed over time, the nervous system adapts to that speed and begins to perceive it as slower than it actually is. Highway driving at 60 mph genuinely feels slower after 20 minutes than it did at the on-ramp. The speed has not changed. The perception has. This is not imagination. It is a documented effect of sustained velocity on sensory adaptation.

I experienced this directly about two decades ago in Las Vegas, where I rode as a passenger in a two-seat open-wheel race car travelling at 200 mph around a speedway oval. At first the speed was overwhelming. Within a few laps, the sensation had normalized to the point where it felt almost routine. That same evening I developed significant neck pain from the G-forces generated through the banked turns. Newton had been making a very clear point while I was busy feeling comfortable.

The second reason is the difference in perceptual context. A free fall from a height offers visual and vestibular feedback that is unmistakably alarming: the rushing ground, the sensation of acceleration, the absence of any protective structure. A car collision at the same terminal velocity happens inside a familiar enclosed space, with a seat, a seatbelt, and windows. The psychological context suppresses the fear response even when the physical forces are equivalent.

The third reason is familiarity. Most of us have driven at highway speed hundreds or thousands of times without incident. That familiarity creates a baseline assumption of safety that is not physically justified by the forces involved.

What the Body Can Tolerate

The human body, when healthy, can absorb a collision of approximately 4 mph with minimal injury. This is roughly the speed of a brisk jog, or the impact of stumbling over a step. At this speed the soft tissues of the spine, the deep muscles, the fascia, the discs, and the ligaments can absorb and distribute the force without significant structural damage.

Above 4 mph, tissue injury becomes increasingly likely. The specific pattern and severity of that injury depend on the direction of the force, the position of the spine at impact, the age and pre-existing condition of the tissues, and whether any protective mechanisms such as bracing or muscle activation were engaged at the moment of impact.

Modern vehicle engineering has made meaningful progress in reducing the forces transmitted to occupants through crumple zones, airbags, seatbelts, and collision detection systems. These technologies extend the time over which deceleration occurs, which reduces the peak force reaching the body. They do not eliminate the injury mechanism. They moderate it.

A rear-end collision at 30 mph in a modern vehicle is not equivalent to a free fall from three storeys without modification. But it is also not a minor event, and treating it as one, particularly in the acute assessment phase, is where the clinical failures in whiplash care begin.

Why This Framing Matters Clinically

The fall-height comparison is not an academic exercise. It is a tool for recalibrating how collisions are perceived by everyone involved in the aftermath: the patient, the clinician, the insurer, and the medicolegal system.

When a patient is told their accident was a low-speed impact and their WAD 1 assessment found no injury, they are receiving a message that is inconsistent with the physics of what their body just experienced. When a clinician dismisses a 25 mph rear-end collision as unlikely to produce significant tissue injury, they are applying a perceptual framework that does not account for what the tissues of the cervical and thoracic spine actually absorbed.

The forces involved in road travel are not small. The human body is not designed to absorb them without consequence. Early recognition of this is the first step toward appropriate investigation and care.

The information in this article is educational and informational in nature. It is not intended as a substitute for professional medical advice, diagnosis, or treatment. If you have been involved in a motor vehicle accident, consult with a qualified healthcare provider to discuss appropriate assessment and care for any injuries sustained.