How to Choose Traffic-Calming Measures Without Moving Congestion Elsewhere

The short answer

A single calming device rarely fixes a corridor; it usually reroutes part of the flow onto whichever parallel street offers the quickest detour. To avoid exporting your problem, start by defining the real goal (lower speed or remove through traffic), measuring volumes and speeds on both the target street and likely alternate routes, matching the measure to the street's function and class, and designing zone-wide rather than street-by-street. Pilot it, re-measure after a month and after six months, and set a boundary-road threshold before you install anything.

Key takeaways

Separate the two goals: speed-reduction measures (humps, raised crossings, chokers, circles) mostly keep cars on the street, while volume measures (diverters, closures, turn restrictions) deliberately redistribute flow — and those are the ones most likely to burden your neighbors.
Whether displacement happens depends less on the device than on whether drivers have a shorter alternative route; check parallel streets in advance, not after complaints arrive.
Functional class decides: speed humps and small traffic circles are inappropriate on arterials and major through routes, while closures and diverters must preserve emergency, transit, and freight access.
Zone-wide, network design beats one-off fixes: closing every entry to a cut-through route (and directing trips to a capable arterial) prevents the traffic from simply leaking into the next residential street.
Not all displaced traffic returns: some trips evaporate as people change time, mode, or destination, and boundary-street surges often fade after a bedding-in period of several months.
A temporary pilot with before/after counts on the subject and adjacent streets — and a pre-agreed boundary threshold — is the only reliable way to tell where the traffic actually went.

Name the real problem: slower speeds or less traffic?

Traffic calming serves two distinct objectives, and each pulls toward different devices. The first is speed: you want drivers who remain on the street to travel more slowly and predictably. That is the job of vertical and horizontal deflections — speed humps and cushions, raised crosswalks and intersections, chicanes, chokers and small traffic circles. The second is volume: you want to remove through (rat-running) traffic that uses a residential street as a shortcut. That is the job of routing restrictions — diagonal diverters, half or full closures, median barriers and forced-turn islands.

Confusing the two produces most of the disappointment. If residents complain about speeding and you install a closure, cars vanish but complaints move next door. If the problem is cut-through traffic and you only slow cars, they stay, and so does the noise and danger. The evidence backs this up: speed-reduction devices on their own can lower daily volumes by an average of roughly a fifth, but the range is enormous — from reductions of over 70% down to no change or even increases, depending on the setting and available alternatives.

So write the objective down as a measurable target before designing: for example, 'cut the 85th-percentile speed from 40 to 30 mph' or 'reduce through traffic on the street by 40%.' The phrasing determines which tools are legitimate and which are not.

Goal 'slow it down': humps/cushions, raised crossings, chokers, chicanes, small circles.
Goal 'remove through traffic': diagonal diverters, half/full closures, forced-turn islands.
Mixed goal: combine groups, but evaluate the effect of each type separately in the counts.

Why a single point measure can just move the queue

Calming does not erase trips; it reroutes them, retimes them, or in a minority of cases removes them altogether. When a street slows or closes, some drivers will switch to whatever parallel or boundary road offers less delay. The availability of that faster alternative — not the device itself — decides how much traffic leaves the street and where it lands. A capacious parallel road absorbs the flow; a narrow residential one simply inherits your problem.

Crucially, not all displaced traffic reappears. Some drivers change departure time, shift to another mode, chain errands together, or drop the trip. In real low-traffic schemes, such as Manchester's Parsonage Road trial, traffic inside the filtered area fell by roughly 50–55%, while the northern boundary road showed no significant reassignment; on another boundary segment traffic rose by about 14% at the start but settled near 6% after six months. That is the 'bedding-in' effect: the network adapts to a new layout over time.

The practical consequence: never accept or dismiss the claim that 'the problem moved next door' on faith. Test it with counts on the target street and on two or three likely diversion routes, collected before work starts and again after it has settled.

Traffic moves where a quicker alternative exists; without one the measure underperforms, with one it can overburden neighbors.
A portion of traffic evaporates, so drops inside a zone usually exceed increases on boundary roads.
Expect a temporary boundary surge right after opening; judge results after a few months, not days.

Match the measure to the street's class and network role

Authoritative guidance links each device to the street's functional classification. Speed humps and small traffic circles, for instance, are rated as inappropriate on arterials and major thoroughfares; they belong on local and residential streets. Width reductions, median islands, parking-based measures and road diets apply more broadly, up to and including collector roads. Routing restrictions — diverters and closures — demand caution wherever emergency response, transit, large vehicles or commercial access matter.

At the district scale, treat calming as a network concept rather than a pile of obstacles. Through streets inside a zone are converted into dead-ends, loops or rings, and the arterial network absorbs the redistributed transport load. In this logic, calming works by sharpening the functional hierarchy of streets — it does not ban driving 'in general.' Where no capable arterial exists, closing one street will very likely overload the next one.

So before choosing hard measures, confirm that the zone is served by an arterial that can actually take the transit trips. If it is not, strengthen the arterial first (signal coordination, protected turn lanes) or confine yourself to speed and width measures that do not push traffic off the street entirely.

Arterial / major route: avoid humps and small circles; consider medians, road diets, lane reconfiguration.
Collector / residential collector: wider menu — chokers, raised crossings, chicanes, careful diverters.
Local residential: full toolbox including closures, provided residents, emergency and maintenance access survive.

Design at the network level so traffic cannot simply leak

The surest way not to hand congestion to a neighbor is to deny the flow a single easy exit. Close one cut-through and leave every adjacent residential passage open, and the rat-running merely redistributes along them. That is why measures are installed as a package: filters and closures are placed so that a through route across a residential area becomes impossible, not merely less convenient. Package, network-level design is what distinguishes a genuine calming 'zone' from a scatter of humps.

Two questions drive the layout. First, where does the transit go? In the ideal case, onto an arterial with spare capacity and coordinated signals. Second, how does local access survive? Residents, emergency services, freight and public transport need a guaranteed — if not the shortest — path to their destinations. A good scheme makes through travel unattractive to strangers without punishing people who genuinely need to be there.

A complementary tool is the road diet on approach streets: reconfiguring an undivided four-lane road into three lanes with a center two-way left-turn lane. Done where volumes warrant, it cuts total crashes by roughly a fifth to a half, calms speeds, and often carries as much traffic as the old layout, so drivers have less reason to hunt for detours. Feasibility depends on volume, so check that the existing lane count exceeds what the traffic actually needs.

Block the through route at every entry to the zone, not just the one 'bad' street.
Send transit toward an arterial with reserve capacity and coordinated signals.
Preserve access for residents, emergency services and transit, or you trade quiet for conflict.
Use road diets to calm a corridor without necessarily cutting its carrying capacity.

Pilot, measure before and after, then adjust

Even a well-reasoned design is a hypothesis. Start with a temporary pilot built from delineators, removable rubber humps, planters and temporary markings. It is inexpensive, reversible, and shows real driver behavior and community reaction before permanent money is spent. Document the 'before': daily volumes and 85th-percentile speeds on the subject street and on likely diversion routes, plus journey times along key corridors.

After launch, repeat the counts at two points — roughly one month and six months out. A measurement taken immediately after opening shows the worst-case redistribution during bedding-in; the later one shows the settled picture. Compare peak hours as well as daily averages, because congestion that 'moves' does so most visibly at the peak. If growth on a diversion route exceeds a threshold you agreed on in advance (say +10–15% over baseline), do not rip everything out: add a measure on the diversion street itself, introduce signal coordination, or refine the geometry.

This mirrors the recommendation that agencies monitor the operational and safety effects after any reconfiguration and make changes to keep flow and safety acceptable. Monitoring also supplies the data you will need if the scheme is challenged.

Count 'before' and 'after' on the subject street and two or three adjacent streets — only that shows where the flow went.
Run a temporary pilot before permanent construction.
Evaluate at roughly one and six months, allowing for the bedding-in period.
Agree a tolerable boundary-road threshold before launch, not after complaints.
If a diversion route exceeds the threshold, adjust the layout rather than abandoning the whole scheme.

Pitfalls, jurisdiction and honest limits

Common failures repeat across cities. Treating one street while ignoring the network; choosing a device 'by eye' without speed and volume data; declaring success from the subject street alone while never watching the neighbors; and expecting calming to dissolve all congestion. Calming is a safety and livability tool, not a capacity-increasing one — its job is to make streets calm, not to add throughput.

There are also hard limits. Speed measures rarely reduce volume on transit streets with a convenient bypass. Closures can complicate emergency response and commercial delivery. Poorly designed elements — too high, too steep, wrongly profiled — are uncomfortable or dangerous for cyclists and motorcyclists and can damage vehicles. Every choice is a trade-off among safety, access and comfort, and no device satisfies everyone. State your priorities and your jurisdiction's standards (for example, national guidance on device design and placement) up front, and revisit them during evaluation rather than improvising after the fact.

Finally, keep expectations realistic about scope. Whether you are calming a single local street, a school approach or a neighborhood, success should be defined as measurable speed and volume outcomes on the target street without an unacceptable increase on adjacent roads — not as an end to traffic everywhere.

Never judge success from one street alone; watch the network and peak hours.
Do not replace data with guesswork — without a 'before' baseline you cannot defend the outcome.
Remember the purpose is safety and quality of place, not clearing roads at any cost.
Check the effects on cyclists, motorcyclists, emergency vehicles and freight, and comply with your jurisdiction's design standards.

Put it into practice

Calming measure selection and displacement check

A working checklist to take a scheme from 'a hump would be nice' to a defensible, measured decision. Fill it in at concept, pilot and evaluation stages so that the choice rests on data and thresholds, not on whoever argues loudest.

Write a measurable objective: a speed target (e.g., 85th percentile at or below 30 mph) and/or a volume target (cut through traffic by N%).
Collect 'before' data: daily volumes and speeds on the target street plus 2–3 likely diversion routes; note peak-hour conditions.
Confirm the street's functional class and constraints: emergency access, transit route, freight movement, on-street parking.
Verify that a capable arterial can absorb diverted transit; if not, strengthen it first or avoid hard closures.
Select device type by objective: speed devices (humps, chokers, raised crossings) or volume devices (diverters, closures).
Plan the package across every entry to the zone so no cut-through 'hole' remains.
Run a temporary pilot and pre-agree the acceptable boundary-road growth threshold (e.g., +10–15% over baseline).
Measure 'after' at roughly one and six months on the target and adjacent streets, separating peak-hour from daily figures.
If a diversion route exceeds the threshold, adjust the layout or add a measure there rather than demolishing the scheme.
Record residents' and agency priorities at the start so they anchor the final evaluation.

Questions people ask

Which traffic-calming measures reduce speed, and which actually cut traffic volume?

Speed is reduced by vertical and horizontal deflections: speed humps and cushions, raised crosswalks, chokers, chicanes and small traffic circles. These mostly keep cars on the street while slowing them. Volume (through traffic) is reduced by routing restrictions: diagonal diverters, half and full closures, median barriers and forced-turn islands. If your problem is rat-running, speed devices alone will not solve it; if your problem is speeding on a road that should keep its traffic, hard closures are the wrong tool.

Why does congestion move to the next street, and can that be avoided?

Calming does not cancel trips; it reroutes them. When a street slows or closes, drivers switch to the parallel or boundary road that offers the least delay, which is why the problem can reappear next door. You cannot eliminate redistribution entirely, but you can manage it: install measures as a package across all entries to a zone, route through traffic toward a capable arterial, and count adjacent streets before and after. Evidence also shows that a share of traffic evaporates and that boundary surges often fade after several months of bedding-in.

How long should I wait before judging whether a scheme displaced congestion?

A single count right after opening is misleading, because the network takes time to adapt and boundary roads can show a temporary surge that later subsides. The recommended approach is two evaluation points — roughly one month and six months after launch — compared against a 'before' baseline. Analyze peak hours as well as daily averages, since displacement shows up most strongly at the peak. Only the settled picture tells you whether the scheme moved the problem or actually reduced trips.

Are speed humps appropriate on an arterial or major transit street?

No — authoritative guidance rates speed humps and small traffic circles as inappropriate for arterials and major thoroughfares; they belong on local and residential streets where slower operating speeds and low volumes are expected. On busier corridors, agencies typically use measures such as medians, road diets and lane reconfiguration, which calm traffic without creating abrupt obstacles. Always match the device to the street's functional classification and your jurisdiction's design standards.

Should I run a pilot before installing permanent calming measures?

Yes. A temporary pilot built from delineators, removable rubber humps and temporary markings is inexpensive and reversible, and it reveals real driver behavior and community reaction before permanent investment. Collect before-data first, then after-data on both the subject and adjacent streets. Official guidance also stresses monitoring the operational and safety effects after any reconfiguration and adjusting as needed. Check your jurisdiction's standards for device design, placement and approval before you finalize anything.

Sources and further reading

Sources were checked when this page was generated. Confirm changing dates, rules and prices with the original publisher.

Traffic Calming ePrimer (module overview)U.S. Federal Highway Administration
Traffic Calming ePrimer – Module 3: Toolbox of Individual Traffic Calming MeasuresU.S. Federal Highway Administration
Traffic Calming ePrimer – Module 4: Effects of Traffic Calming Measures on Motor Vehicle Speed and VolumeU.S. Federal Highway Administration
Road Diet Informational Guide – Executive SummaryU.S. Federal Highway Administration
Parsonage Road safer streets – Evaluation Report executive summaryManchester City Council
Методические рекомендации по разработке и реализации мероприятий по организации дорожного движения. Методы «успокоения движения»Министерство транспорта РФ (текст на Meganorm)
Технические и планировочные приемы успокоения движенияИркутский национальный исследовательский технический университет