Traffic Calming That Actually Works Speed Humps, Chicanes & Narrowing

Small drops in impact speed dramatically cut the risk of people being killed or seriously injured. A driver hitting someone at 30 mph has a vastly higher chance of causing severe harm than at 20 mph; modern syntheses show fatal or severe injury risk rises steeply beyond 20–25 mph—precisely the range most neighborhood streets target.

That’s why engineering that physically reduces speeds is the backbone of Vision Zero strategies.

What “works” in 2025: three families of countermeasures consistently earn top marks in recent U.S. and international guidance—vertical deflection (speed humps, speed tables/cushions), horizontal deflection (chicanes, lane shifts), and lane narrowing/road diets that set a safe target speed and remove excess through-capacity.

The short list: what each tool does best

Speed humps (and cousins: tables, cushions)

• What they are: Parabolic vertical elements, typically 3–4 in high and 12–14 ft long (not short “bumps”). Designed for low-volume, low-speed streets.
• Target speeds & spacing: Properly designed corridors see operating speeds around 15–20 mph. To hold 85th-percentile speeds in the 25–30 mph range, place devices roughly 260–500 ft apart (closer spacing = lower speeds).
• Crash results: Multiple studies link humps to fewer injuries—e.g., 53–60% lower odds of child injury where humps are present; a Canadian cohort found a 44% drop in child pedestrian collisions post-installation.
• 2025 evidence: New work continues to show large speed reductions (≈46–52% depending on hump profile) when designs meet spec.
• Cost reality: Asphalt humps commonly $4k–$5k each (U.S. city examples), tables $5k–$7k, with local variance; some quick-build rubber humps can be ≈1/10 the price of asphalt (useful for pilots).
• Emergency response: Expect ~3–10 s delay per device for fire/EMS—why many cities prefer speed cushions (gapped), which large axles can straddle with near-zero delay.

Pro tip: Use sinusoidal or bus-friendly tables on transit/ambulance routes; reserve classic humps for streets not on primary response corridors.

Chicanes (lateral shifts)

• What they are: Alternating curb extensions/islands create a gentle S-curve, forcing drivers to steer (and slow).
• Speed effects: Typical 5–13 mph reductions (context-dependent). Historic U.S. case series report ~8–12 mph drops on neighborhood streets.
• Design notes: Trapezoidal islands tend to calm speeds better than semicircles and can be cheaper when they leave gutter drainage intact.
• Costs: Widely variable—from $5k–$18k for simple raised deflection to $25k+ where drainage/curb reconstruction is needed.
• Operations: Usually low emergency delay; snow/streetsweeping and driveway access need attention during design.

Pro tip: Pair chicanes with center islands and edge markings so the visual narrowness matches the geometry—drivers brake earlier and more consistently.

Lane narrowing & road diets (a.k.a. roadway reconfiguration)

• What they are: Convert a 4-lane undivided to 3 lanes (2 through + center turn lane) with space for bike lanes/refuges/parking; or simply restripe to narrower lanes that support a lower target speed.
• Crash benefits: FHWA reports ~19–47% overall crash reductions after classic road diets on suitable 4-lane undivided corridors (≤~25k ADT).
• Lane width findings (2023–2025): New national analyses suggest 9–10 ft lanes on 20–35 mph urban streets can be safer than wider lanes; a New York corridor that narrowed to 10 ft saw 60% fewer injury crashes and 90% fewer serious injuries. (Context matters—don’t squeeze heavy bus corridors below 10 ft without analysis.)
• Cost: When bundled with resurfacing, diets/restriping can be low-cost (often tens of thousands per mile), versus six-figure geometric reconstructions.

Pro tip: Design to target speed (not “85th percentile”) and select lane widths that self-enforce it—10 ft is a strong default for 25–35 mph contexts.

Specs that practitioners actually use (and why)

• Dimensions that deliver: Speed humps at 3–4 in height and 12–14 ft length reduce peaks without harsh jolts; tables use a flat deck; cushions split the hump into pads with wheel gaps so emergency axles can straddle.
• Spacing that sticks: Install vertical devices in series, ~260–500 ft apart; one lonely hump won’t hold speeds between devices.
• Sight distance & lighting: First device visible ≥250 ft; avoid sharp curves; sign and mark per MUTCD so drivers read the street right.
• Pick the right street: Humps/tables for local & neighborhood collectors; avoid on primary emergency or major transit routes unless using cushions or tables and coordinating with fire/EMS.

What to measure (and show to your council)

• Operating speed: 85th percentile and high-end outliers (e.g., % over 30 mph).
• Crashes & injury severity: aim to cut KSI (killed/seriously injured) rates. Road diets typically deliver ~19–47% overall crash reductions; 20 mph zones (with physical calming) have shown ~42% casualty drops in large city analyses.
• Emergency response: track modeled and field-tested delay (~3–10 s per device for humps is common; cushions ≈ 0–2 s).
• Mode share & perception: after diets and calming, look for higher walking/biking and perceived safety.

Quick-build vs. permanent: where to start

• Pilot fast with paint-and-post chicanes, rubber humps, or striping diets to prove benefits, then convert to permanent concrete/curb if targets are met. Rubber kits cut upfront cost (≈1/10th of asphalt).
• Bundle with resurfacing to keep costs low for diets.

2025 cheat sheet — measures, impacts, costs & notes

Measure	Typical speed impact	Typical crash impact	Cost (order of magnitude)	Emergency response	Design notes
Speed hump	Operating speeds ≈ 15–20 mph when used in series; –8–12 mph vs baseline typical	Child injury odds ↓ 53–60%; child pedestrian collisions ↓ 44% in cohort studies	$4k–$5k/hump (asphalt); $5k–$7k/table; rubber ≈ 1/10 asphalt	Delay ≈ 3–10 s/device; consider cushions on response routes	Height 3–4 in, length 12–14 ft; space 260–500 ft in series
Speed cushions	Similar to humps for cars	Similar where used	Similar to tables	≈0–2 s delay (axle straddle)	Use on bus/EMS streets
Chicanes	–5–13 mph typical	Context-dependent; effective on locals	$5k–$18k (simple) to $25k+ (with drainage/curb)	Low	Favor trapezoidal islands; maintain drainage
Lane narrowing	Aligns with target speed; 9–10 ft lanes support 20–35 mph	Corridor examples: injury crashes ↓ 60% after narrowing to 10 ft (context-specific)	Low (re-striping) to moderate (curb work)	None	Avoid <10 ft on heavy bus/truck routes without study
Road diet (4-to-3)	Calms and self-enforces speeds	Overall crashes ↓ ~19–47% on suitable 4-lane undivided roads	$25k–$40k+/mi restripe; geometric work higher	None	Best at ≤25k ADT; adds turn lane, bike lanes, refuges

Data and figures synthesized from 2023–2025 federal/state guides and peer-reviewed studies.

How to choose for your street (a 5-step playbook)

1. Set the target speed (20–30 mph for most urban locals/collectors). Design for it, don’t hope enforcement will carry the day.
2. Match the tool to the street:
   • Local streets (not primary EMS): humps/tables in series.
   • Transit/EMS routes: cushions or tables, or chicanes where geometry allows.
   • Overwide collectors/arterials: lane narrowing or road diet .
3. Design to spec: get the geometry right—hump profiles, chicane tapers, and spacing drive outcomes.
4. Pilot with quick-build; gather before/after speed data, KSI trends, and EMS delay measurements.
5. Make it permanent where targets are met; adjust for curb, drainage, and winter ops.

Common concerns (and honest answers)

• “Won’t this slow fire trucks?” Classic humps can add ~3–10 s each; cushions and “offset tables” cut that to near-zero while still calming drivers. Coordinate device placement and route selection with your fire chief.
• “Do narrower lanes cause more crashes?” New national work shows 9–10 ft lanes on 20–35 mph urban streets are not more dangerous—and often safer; still analyze corridors with heavy bus/truck volumes before narrowing below 10 ft.
• “Isn’t it cheaper to just enforce?” Engineering self-enforces all day, every day—no overtime required. Diets during resurfacing can be low- or no-additional-cost, while vertical/horizontal deflection remains relatively affordable compared with crash costs.

Implementation details you shouldn’t skip

• Signing & markings: Use MUTCD-compliant warning signs for vertical elements; mark gore areas at chicanes; refresh paint after winters.
• Drainage checks: Prefer edge islands for chicanes when gutters must remain open; avoid ponding at humps/tables.
• Equity lens: Prioritize streets with historical high-injury burdens; many big casualty drops in 20 mph zones came from focusing on vulnerable users and places.
• Performance dashboard: Publicly track 85th-percentile, % over 30 mph, KSI, and EMS delay, updated quarterly.

FAQs