Speed bumps are important because they force drivers to slow down in areas where pedestrians are most vulnerable, and even a small reduction in speed dramatically changes whether a crash is survivable. A pedestrian hit by a car traveling 20 mph has a 90 percent chance of surviving. At 30 mph, that survival rate drops below 50 percent. Speed bumps exist to keep vehicles in that lower, survivable range.
How Much They Actually Slow Traffic
Speed bumps, humps, and speed tables all work by creating a vertical jolt that makes driving above a certain speed uncomfortable. The specific design determines how much slowing occurs. According to Federal Highway Administration data, speed humps typically bring the majority of drivers down to the 25-to-27 mph range, regardless of the posted speed limit or how fast drivers were going before installation. Speed tables, which are wider and flatter, produce slightly higher speeds in the 30-to-32 mph range.
That 25-to-27 mph sweet spot is significant. It sits right in the zone where pedestrian crashes are far more likely to be survivable. Without the humps, residential streets that feel wide and open can easily invite speeds of 35 to 45 mph, pushing well into the range where a pedestrian strike becomes fatal 85 percent of the time.
The Effect on Pedestrian Crashes
Slowing cars down translates directly into fewer people getting hit. A study in Los Angeles found a 37.5 percent reduction in collisions between vehicles and pedestrians under 21 after a single speed hump was installed. Research in Oakland found that speed humps were associated with a 53 to 60 percent reduction in the odds of children being injured or killed by a car in their neighborhood. A Toronto study observed a 22 percent decrease in pedestrian-vehicle incidents overall, and a South African study that installed speed humps near 34 schools across two cities measured a 23 percent improvement in pedestrian safety.
These numbers are consistent enough across different countries and study designs to be reliable. The general expectation, based on crash reduction factors used by traffic engineers, is that speed humps produce roughly a 50 percent decrease in pedestrian-involved collisions. Children benefit the most because they’re the least predictable pedestrians: they dart into streets, play near roads, and are harder for drivers to see. Slower vehicles give drivers more reaction time and shorter stopping distances.
Why Small Speed Differences Matter So Much
The relationship between speed and pedestrian fatality risk isn’t linear. It’s exponential. Going from 20 mph to 30 mph doesn’t just increase the danger a little. The fatality risk jumps from about 5 percent to 45 percent. At 40 mph, it reaches 85 percent. This means the difference between a driver cruising through a neighborhood at 35 mph and one forced to slow to 25 mph by a speed hump can be the difference between a fatal crash and one where the pedestrian walks away with bruises.
Stopping distance explains part of this. A car traveling 20 mph needs roughly half the distance to stop compared to one going 30 mph. But the bigger factor is the energy transferred on impact. Kinetic energy increases with the square of speed, so a car moving 40 mph carries four times the energy of one moving 20 mph. That energy gets absorbed by the human body.
Bumps, Humps, Tables, and Cushions
The terms get used interchangeably, but these are distinct designs with different purposes. Speed bumps are the shortest and most aggressive: typically 2 to 4 inches tall and only a few inches long. They create a sharp jolt and are most common in parking lots where target speeds are under 10 mph. Speed humps are longer and more gradual, spanning about 12 to 14 feet. They’re the standard choice for residential streets because they slow traffic effectively without being jarring at 25 mph. Speed tables are even wider, essentially a flat-topped hump that can double as a raised crosswalk.
Speed cushions are a newer variation designed to solve one of the biggest criticisms of speed humps. They’re humps or tables with gaps cut into them, spaced to match the wider wheelbase of fire trucks and ambulances. Emergency vehicles can straddle the raised portions and pass through without slowing down, while regular passenger cars still have to drive over them. The National Association of City Transportation Officials recommends using cushions on major emergency access routes for exactly this reason.
The Emergency Response Tradeoff
One of the most common arguments against speed bumps is that they delay ambulances and fire trucks. This concern has real data behind it. A standard speed hump costs an ambulance roughly 5 to 10 seconds per hump, depending on the vehicle type and the speed it was traveling. Heavier, multi-victim ambulances lose the most time, around 9 to 10 seconds per hump. Lighter van-style ambulances lose about 5 to 8 seconds each.
On a street with several humps, those seconds add up. A route with five humps could add 30 to 50 seconds to a response. In cardiac arrest, where every minute without treatment reduces survival odds, that’s a legitimate concern. This is precisely why speed cushions were developed and why cities increasingly use them on streets that serve as primary emergency routes. The goal is to get the pedestrian safety benefits without compromising response times for the people who need ambulances most.
Effects on Emissions and Noise
Speed bumps create a repeated cycle of braking and accelerating that affects both air quality and noise. Every time a driver slows for a hump and then speeds back up, the engine works harder than it would at a constant speed. Research measuring instantaneous emissions found that speed humps significantly increase carbon dioxide, nitrogen oxides, and particulate matter, especially on roads with light traffic where vehicles are free to accelerate aggressively between humps. The more humps on a road, the more pronounced the stop-and-go effect becomes, compounding the emissions increase.
On busier roads where traffic is already stop-and-go, the additional impact of humps on emissions is smaller because vehicles aren’t reaching high speeds between them anyway. Noise follows a similar pattern. The braking, acceleration, and occasional scraping of low-clearance vehicles creates sound disturbances that residents living directly next to humps sometimes find worse than the traffic noise they were trying to reduce. Proper spacing and placement help minimize these effects, but they’re a real consideration in urban planning.
How They Compare to Other Traffic Calming
Speed humps aren’t the only way to slow traffic. Chicanes, which are curb extensions that force drivers to weave through a narrowed road, achieve roughly the same reduction in average speed. However, research comparing the two approaches found an important difference: chicanes produce more variation in how fast individual drivers go. Some drivers slow down significantly while others try to maintain speed through the curves. Speed humps, by contrast, force nearly everyone to the same speed because the physical discomfort of hitting a hump too fast is immediate and universal.
That consistency matters for safety. When all vehicles are moving at similar speeds, the road is more predictable for pedestrians, cyclists, and other drivers. Speed variation, where one car is going 20 mph and the next is going 35, creates more dangerous conditions than a uniform flow at 27 mph. This is one reason speed humps remain the most widely used traffic calming tool in residential areas despite their drawbacks. They work reliably, they’re relatively inexpensive to install, and their effect on driver behavior is immediate and hard to ignore.