The gaps in speed bumps are there primarily to let emergency vehicles and buses pass through without slowing down, while still forcing regular cars to drive over the raised sections. These gapped versions are technically called “speed cushions,” and their design is surprisingly precise, built around the fact that fire trucks and ambulances have much wider axles than passenger cars.
How Speed Cushions Work
A speed cushion is a speed bump or speed table with cutouts (gaps) placed so that vehicles with wide wheelbases can straddle the raised portions, rolling their wheels through the gaps on either side. A typical passenger car has a narrower track width, so its wheels land squarely on top of the raised sections and the driver has to slow down. A fire truck, with front wheels spaced about 5.5 to 5.8 feet apart and an overall width approaching 10 feet, can pass its wheels on either side of the cushion and maintain speed.
This solves one of the biggest complaints about traditional speed bumps: they delay emergency response. Every second counts when a fire truck or ambulance is racing to a call, and a neighborhood with six or eight continuous speed bumps can add meaningful time to that trip. Speed cushions keep traffic calming in place for everyday drivers without creating obstacles for the vehicles that need to move fastest.
Emergency Vehicles and Buses Get Priority
The gap spacing is engineered around specific vehicle dimensions. A standard fire engine pumper is about 9 feet 9 inches wide, with interior rear wheel spacing of roughly 4.1 feet. An aerial platform (ladder truck) is about 10 feet wide. Ambulances run about 9 feet wide with front wheels spaced around 5.8 feet apart. All of these are significantly wider than a sedan or SUV, which is why the cutouts work selectively.
Public transit buses benefit from the same principle. Their wide axles allow them to straddle the cushions, keeping passengers from getting jostled on every block of a bus route. This matters for rider comfort, but also for reducing wear on bus suspensions over thousands of daily trips.
Cyclists Benefit Too
The gaps serve a second, less obvious purpose: they give cyclists a smooth path through. A person on a bike can ride between the cushions without hitting the raised surface at all. The Federal Highway Administration notes that speed cushions are “a unique measure that can accommodate the mobility needs of both bicyclists and fire vehicles.” For anyone on two wheels, hitting a traditional speed bump at even moderate speed can be jarring or dangerous, especially on a road bike with thin tires. The gaps eliminate that problem entirely.
Why Precision Matters in the Design
Getting the gap width right is critical. Too wide, and regular cars can also straddle the cushions, defeating the purpose. Too narrow, and emergency vehicles clip the raised sections. This is one reason many cities use prefabricated rubber or plastic modular units rather than building cushions out of asphalt. Modular cushions are manufactured to exact dimensions and snap together like interlocking puzzle pieces, ensuring consistent gap widths across every installation. Asphalt cushions can work, but achieving the precise measurements needed is harder with poured material that has to be shaped on-site.
The modular approach also makes repairs and adjustments simpler. If a section gets damaged or a city needs to reconfigure a street, individual pieces can be swapped out without tearing up the road.
Drainage Is a Practical Bonus
Continuous speed bumps that span the full width of a road can trap water along their uphill side, especially on streets without strong cross-slopes. The gaps in speed cushions allow stormwater to flow through naturally, reducing puddle formation and the risk of ice buildup in colder climates. This isn’t usually the primary reason gaps are included, but it’s a real functional advantage that road engineers appreciate, particularly in areas prone to heavy rain or freeze-thaw cycles.
Why Not Just Use Gaps Everywhere?
Speed cushions aren’t always the right choice. On narrow residential streets where speeding is severe, a full-width speed bump or speed hump forces every vehicle to slow down, with no exceptions. Some neighborhoods prefer that level of enforcement. Speed cushions also rely on drivers keeping their wheels in the normal lane position. A driver who deliberately swerves to thread their compact car through the gaps can partially avoid the speed reduction, though the cushion still causes enough disruption to discourage high speeds.
Cities typically use speed cushions on streets that serve as emergency routes or bus corridors, where full-width bumps would cause unacceptable delays. On quieter cul-de-sacs or streets rarely used by emergency vehicles, traditional bumps or humps remain common. The choice comes down to balancing speed reduction for cars against access needs for larger vehicles.