Yes, speed kills, and the relationship is not linear. Small increases in vehicle speed produce dramatically larger increases in the risk of dying in a crash. In 2023, speeding was a contributing factor in 29% of all traffic fatalities in the United States, making it one of the leading causes of death on the road alongside impaired and distracted driving.
The physics behind this are straightforward: kinetic energy increases with the square of speed. Double your speed and you quadruple the energy that must be absorbed by your vehicle, your body, or the person you hit. That basic math shapes everything from how far your car travels before stopping to whether a pedestrian survives being struck.
What Happens to Pedestrians at Different Speeds
The difference between survivable and fatal for a pedestrian comes down to a remarkably narrow speed range. At 20 mph or below, a pedestrian has roughly a 90% chance of surviving a crash. At 30 mph, that survival rate drops below 50%. By 40 mph, the fatality risk climbs to about 85%.
To put that another way: increasing from 20 to 30 mph, just a 10 mph jump, raises the risk of killing a pedestrian from around 5% to 45%. That’s a ninefold increase in lethality from a speed change most drivers wouldn’t think twice about. This is why urban speed limits cluster around 25 mph in cities that have studied pedestrian safety closely.
Why Stopping Distance Grows So Fast
Speed doesn’t just make crashes more violent. It also makes them harder to avoid. At 50 mph, a vehicle needs 221 feet to come to a complete stop, accounting for both the driver’s reaction time and the braking distance. At 60 mph, only 20% faster, the stopping distance jumps to 292 feet, a 44% increase. That extra 71 feet is roughly the length of four parked cars, and it’s often the difference between a near-miss and a collision.
Reaction time is fixed. It takes the average driver about 1.5 seconds to recognize a hazard and move their foot to the brake. During that delay, a car traveling 50 mph covers roughly 110 feet before braking even begins. At higher speeds, you’re covering more ground in that same mental processing window, which means hazards that would have been avoidable at lower speeds become unavoidable.
The Fourth-Power Rule
Traffic safety researchers use something called Nilsson’s Power Model to predict how changes in average speed affect crash outcomes. The core finding: fatal crashes increase with the fourth power of any increase in mean speed. If average speed on a road rises by 10%, fatal crashes rise by roughly 46%. A 20% increase in speed nearly doubles fatalities.
This fourth-power relationship explains why even modest speed reductions save lives at the population level. It also explains why high-speed roads are so much deadlier per mile traveled than low-speed ones. The relationship between speed and serious injury follows a similar curve, though slightly less steep, at roughly the third power.
Your Car’s Safety Features Have a Speed Limit
Modern vehicles are engineered to protect you in a crash, but that protection has a ceiling. The Insurance Institute for Highway Safety tested identical vehicles at three impact speeds: 40, 50, and 56 mph. Only the 40 mph test earned a “good” safety rating. Both the 50 and 56 mph tests earned “poor” ratings, with the higher-speed crashes overwhelming the vehicle’s crumple zones and producing dangerous injury measures to the head, chest, and legs.
At 56 mph, the car’s occupant compartment was heavily compromised, meaning the structural cage designed to protect you had partially collapsed. The kinetic energy at those speeds simply exceeded what the vehicle’s energy-absorbing structures could handle. Airbags and seatbelts help enormously, but they’re designed around a specific energy range. Exceed that range and survival odds drop sharply, even in a five-star-rated car.
What Happens When Cities Slow Down
New York City provided one of the clearest real-world tests of whether lowering speed limits works. In November 2014, as part of its Vision Zero initiative, the city dropped the default speed limit from 30 to 25 mph on streets that didn’t have posted signs. Researchers at the City University of New York studied the results and found a 38.7% decline in casualties and a 35.8% reduction in crashes on the affected streets compared to streets where the limit didn’t change.
One nuance worth noting: the reduction was almost entirely driven by a drop in injuries rather than fatalities. Fatalities on the treated streets showed no measurable change. This likely reflects the fact that fatal crashes at urban speeds are relatively rare events, making statistical shifts harder to detect in a short study window. The large drop in injuries, though, confirms the core principle. Lower speeds mean less energy in a crash, which means less damage to human bodies.
Speed Is Deadlier Than Most Drivers Realize
People routinely underestimate speed’s role in crashes because the relationship is invisible in everyday driving. Going 35 in a 30 zone feels normal. It doesn’t feel 40% more dangerous. But the physics don’t care about perception. That extra 5 mph lengthens your stopping distance, narrows your reaction window, increases the energy of any collision, and pushes the impact closer to the threshold where your car’s safety systems can no longer protect you or the person you hit.
The consistent finding across decades of traffic safety research, from pedestrian impact studies to city-wide speed limit changes to crash-test engineering, is the same: speed is not just a contributing factor in severe crashes. It is the multiplier that determines whether a crash is minor, serious, or fatal.