ITE and FHWA research shows properly placed speed humps cut 85th-percentile vehicle speeds by 20 to 30 percent, drop traffic volume on the calmed segment by an average of 18 percent, and reduce crash frequency by about 13 percent. Effectiveness varies with hump profile, spacing, and the speeds on adjacent through-streets. Below: the research, plus the design choices that decide whether a hump actually moves the speed needle.
Do speed humps actually work?
Yes -- but only when the design matches the calming target. A 12-foot parabolic asphalt hump on a residential street with a 25-mph posted speed reliably drops the 85th-percentile speed to 18 to 20 mph. A single hump on a long block recovers half the speed loss within 200 feet. A bumpy under-spec hump triggers complaints rather than calming. The FHWA Traffic Calming ePrimer and the ITE Traffic Calming Manual are the two authoritative summaries Cojo's design team works from.
What the Research Says
Speed reduction
The most-cited measurement is the change in 85th-percentile speed -- the speed at or below which 85 percent of free-flowing vehicles travel. Across multiple FHWA and ITE meta-analyses:
- Single 12-ft hump on residential street: 7 to 9 mph reduction (typically 27 mph baseline drops to 18 to 20 mph)
- Series of 3 humps spaced 250 to 350 ft apart: 8 to 11 mph sustained reduction along the corridor
- Parking lot installation: 5 to 8 mph reduction (smaller delta because baseline speeds are lower)
The reduction is durable -- studies measuring speeds 1, 3, and 5 years post-install show no decay as long as the hump is maintained.
Volume reduction
Speed humps redistribute traffic. Drivers who consider the calmed street a shortcut migrate to alternate routes. The FHWA Traffic Calming ePrimer cites typical volume reductions of 12 to 25 percent on segments with humps, with a meta-average around 18 percent. The flip side: adjacent streets see 5 to 10 percent volume increases unless they too are calmed.
Crash reduction
The CMF Clearinghouse maintained by FHWA lists crash modification factors for speed humps. The pooled estimate is a 13 percent crash reduction, with stronger reductions for pedestrian-involved crashes (around 22 percent) than for vehicle-only crashes.
Pedestrian and resident perception
Beyond measurable metrics, surveys consistently show 70 to 85 percent of residents on a calmed street report feeling safer letting children walk or bike. The Oregon DOT residential traffic-calming guidance treats resident-perception data as a legitimate program metric alongside speed numbers.
What Determines Whether a Hump Is Effective?
Cojo's installation files show four design variables that drive 80 percent of the effectiveness variance.
1. Profile shape
Parabolic profiles deliver the best balance of speed reduction and ride quality. A flat-top "table" profile lets buses pass at 25 mph but is gentler. A triangular profile is harshest and tends to generate complaints rather than compliance. The ITE Traffic Calming Manual defaults to parabolic for general residential use.
2. Height
3 to 4 inches at the crown. Below 2.5 inches the hump fails to register; above 4 inches the damage risk rises. See our speed hump dimensions reference for the full spec.
3. Spacing
A single hump produces a "speed cup" -- vehicles slow at the hump and accelerate back to baseline within 200 to 400 feet. A series of humps spaced 250 to 350 feet apart maintains the speed reduction across the corridor. See speed hump spacing for the full guidance.
4. Approach and departure conditions
Long sight lines and high posted speeds on the upstream street reduce hump effectiveness. The closer a hump is to a stop sign, all-way intersection, or other natural slow point, the more reliable the calming.
Speed Humps vs Other Calming Devices
| Device | Typical speed reduction | Volume reduction | Emergency-vehicle penalty |
|---|---|---|---|
| Speed bump (3-ft) | 10 to 12 mph | 25 to 40 percent | High (jolts ambulance gurneys) |
| Speed hump (12 to 14-ft) | 7 to 9 mph | 12 to 25 percent | Moderate (slows fire trucks 5 to 10 seconds per hump) |
| Speed cushion | 6 to 8 mph (cars) | 8 to 20 percent | Low (wheel-track gaps) |
| Speed table (22-ft) | 5 to 7 mph | 5 to 15 percent | Low |
| Raised crosswalk | 6 to 8 mph | 5 to 12 percent | Low |
| Chicane / curb extension | 4 to 8 mph | 5 to 15 percent | Low |
Are Speed Humps Effective in Parking Lots?
Yes, but the metrics shift. Parking-lot baseline speeds are usually 10 to 15 mph -- the absolute reduction is smaller (5 to 8 mph), but the percentage reduction is similar to street installs. The bigger benefit is behavioral: a hump in a parking lot signals to drivers that the lot is a low-speed pedestrian zone. Cojo's installs in school-pickup loops and retail-frontage lots routinely show 30 to 40 percent reductions in pedestrian-near-miss incidents reported by property managers.
When Are Speed Humps Less Effective?
Five conditions where Cojo recommends a different device:
- Fire-access roads -- use speed cushions instead
- Bus routes -- use speed tables
- Mid-block pedestrian crossings -- use raised crosswalks (combine calming with crossing)
- Long blocks (above 1,000 feet) -- a single hump provides minimal corridor benefit; budget for a series or use chicanes
- High-grade streets (above 8 percent slope) -- humps shed water poorly and ice in winter; use horizontal calming (curb extensions, narrowed lanes)
Cojo's Bend Effectiveness Audit
In November 2025 Cojo installed a series of 3 humps on a residential corridor in NorthWest Crossing in Bend, spaced 280 feet apart. A pre-install speed study by the city's traffic engineering office measured the 85th-percentile speed at 31 mph against a 25-mph posted limit. A 60-day follow-up study after install measured 22 mph -- a 9-mph reduction sustained across the corridor. Resident complaint volume to the Bend Police Department traffic line dropped to zero in the same window. See the speed hump installation in Bend page for the full case study.
Get a Speed-Calming Recommendation
Cojo's design team can review your street, parking lot, or campus and recommend the right calming device based on speed targets, emergency-vehicle requirements, and pedestrian conditions. Send a site photo and a brief description and we will return a free desk-review note within one business day. See asphalt maintenance services for the install scope and pricing baseline.