A speed hump is 3 to 4 inches tall at the apex, with 3.5 inches as the most common U.S. standard. Below 3 inches, the hump fails to deliver enough physical feedback to slow drivers consistently. Above 4 inches, the hump risks vehicle undercarriage damage and creates emergency-vehicle delays beyond what the Federal Highway Administration recommends. The Institute of Transportation Engineers (ITE) Traffic Calming Manual Chapter 3 codifies this 3 to 4 inch range as the U.S. standard.
Quick height reference
| Hump type | Apex Height |
|---|---|
| Standard residential speed hump | 3 to 4 in (3.5 in typical) |
| Speed cushion | 3 to 3.5 in |
| Speed table (22 ft flat-top) | 3 to 4 in |
| Speed bump (parking lot, 1 to 3 ft) | 2 to 4 in |
Why is the height 3 to 4 inches?
ITE-referenced field studies on traffic-calming effectiveness show a clear height-to-speed-reduction relationship. Below 3 inches, drivers do not feel enough physical feedback to slow; the 85th-percentile speed reduction drops to less than 5 mph. Above 4 inches, the hump generates suspension topping (the suspension fully compresses then bottoms out) at the target speed range, which damages vehicle components and can lift wheels off the pavement.
The 3.5-inch standard is the empirical sweet spot. Most U.S. residential traffic-calming programs (including Portland PBOT, Salem Public Works, Eugene Public Works, and Beaverton Engineering Review) spec 3 to 4 inches and default to 3.5 inches for new installs.
When is a 3-inch hump the right call?
Three scenarios push the height down to 3 inches:
- Bus routes. A 3-inch hump reduces passenger discomfort on transit buses. Some Oregon cities spec 3 inches on streets that carry TriMet, Lane Transit, or Cherriots service.
- Streets with high cycling traffic. Cyclists feel hump height more than cars; lower height plus the sinusoidal profile reduces cyclist discomfort.
- Streets where pre-install 85th-percentile speeds are below 30 mph. Lower starting speeds need less aggressive slowing geometry.
When is a 4-inch hump the right call?
Three scenarios push the height up to 4 inches:
- Streets with very high pre-install 85th-percentile speeds (above 35 mph). Aggressive slowing requires the upper end of the height range.
- Streets with crash history. Documented speed-related crashes justify maximum slowing.
- Streets with high vulnerable-road-user traffic. Schools, parks, and senior-living facilities sometimes get 4-inch humps because the slowing target is more aggressive.
What happens if a hump is taller than 4 inches?
Vehicle damage rises sharply above 4 inches. The Society of Automotive Engineers (SAE) studies on traffic-calming-induced vehicle damage document increased risk of:
- Suspension topping and bottoming
- Oil pan and exhaust system damage
- Bumper scrape on low-clearance vehicles
- Lifted wheels at design speed (loss of traction)
Beyond 5 inches, the hump is no longer a traffic-calming device; it becomes a hazard. The Federal Highway Administration's Traffic Calming ePrimer documents that height excursions above the 4-inch standard create disproportionate risk versus marginal slowing benefit (FHWA ePrimer).
What about emergency-vehicle access at different heights?
Fire trucks, ambulances, and police cars all cross humps. The delay per truck per hump scales with height:
| Hump Height | Fire Truck Delay (per hump) |
|---|---|
| 3 in | 4 to 6 seconds |
| 3.5 in | 5 to 7 seconds |
| 4 in | 6 to 9 seconds |
| 5 in | 8 to 12 seconds |
For streets where fire-response delay is a stated concern, speed cushions (with wheel-track gaps) are the standard alternative.
How is height measured?
The standard measurement is from the surrounding pavement grade to the apex of the hump (the highest point at the centerline). Use a 10-foot straightedge laid across the hump perpendicular to the direction of travel; measure the gap between the bottom of the straightedge and the apex.
For acceptance testing on a new install, most jurisdictions accept +/- 0.25 inch of the spec value. A 3.5-inch spec accepts 3.25 to 3.75 inches measured at the apex. Larger excursions require rework.
How does pavement maintenance affect height over time?
Speed hump height degrades 0.1 to 0.2 inches per year in moderate-traffic conditions due to compaction settlement and surface wear. After 10 years, a 3.5-inch new hump typically measures 2 to 2.5 inches at the apex. Re-profiling (milling and re-screeding) restores the height; this maintenance step is part of the 15 to 25 year asphalt-hump lifespan.
For maintenance details, see the speed hump maintenance guide.
Frequently asked questions
How tall is a standard speed hump? 3 to 4 inches at the apex, with 3.5 inches as the most common U.S. standard per ITE Traffic Calming Manual Chapter 3.
Are 4-inch speed humps illegal? 4 inches is the upper end of the ITE standard. It is not illegal but is reserved for streets with high pre-install speeds or crash history.
Why are some humps taller than others? Different jurisdictions calibrate hump height to local traffic conditions. High pre-install speeds and crash history justify upper-range height; transit and cyclist priority push toward lower-range height.
Will a tall speed hump damage my car? A 3 to 4 inch hump crossed at 15 to 20 mph does not damage standard passenger vehicles. Damage occurs at speeds above 25 mph or at heights above 4.5 inches.
Does the height change for speed cushions? Speed cushions are typically 3 to 3.5 inches at the apex of each pad. The pad height is similar to a hump; the difference is the wheel-track gap geometry, not the height.
Specify the Right Height
Cojo installs speed humps across Oregon at ITE-compliant heights based on site-specific traffic data. Contact Cojo for a site assessment, or see speed hump dimensions for full specs.