Neighborhood Speed Management Program Engineering Toolkit
As part of its mission to enhance the quality of life in Boulder, the city established the Neighborhood Speed Management Program (NSMP) to mitigate the negative effects of speeding traffic on neighborhood streets. Speeding not only contributes to the severity of injuries and damage in crashes, but can also impede travel for people who walk, bike, skate, play, and move in, around, and through Boulder’s neighborhoods.
To help ensure that neighborhoods are safe and comfortable places, the NSMP focuses on education, enforcement, and engineering to slow drivers down. This toolkit focuses on the engineering component of the program, also known as traffic calming. It includes the variety of devices that can be used to physically slow down vehicles. Some of these devices may be more appropriate than others depending on the neighborhood and street conditions, as well as the need for emergency services to use a route.
What is traffic calming?
Traffic calming is an active attempt to control vehicle speeds through the engineering and design of the roadway and traffic calming devices to slow speeding traffic. The Institute of Transportation Engineers (ITE) defines traffic calming as:
“changes in street alignment, installation of barriers, and other physical measures to reduce traffic speeds and/or cut through volumes, in the interest of street safety, livability, and other public purposes.”
The NSMP is a traffic calming program that specifically targets speed reduction. Residents can apply to the city for traffic calming on their neighborhood streets to reduce traffic speeds. More information on the variety and effectiveness of traffic calming devices can be found below.
Purpose
Speed humps and speed cushions are designed to slow vehicles with a vertical change in pavement. They are parabolic in shape, typically between 3 and 4 inches tall and 12 feet wide. They extend to the edge of the gutter on each side of the street, or the edge of the pavement where there is no curb and gutter.
Speed cushions differ from speed humps by including gaps to allow the wheels of larger vehicles such as fire trucks or transit vehicles, or bicycles, to pass through the device unimpeded. The gaps are spaced so that passenger vehicles and small trucks will need to drive over the cushions.
Effectiveness
- Speed humps and cushions can be effective in reducing vehicular speeds up to 20%
- Speed humps are most effective on local, non-emergency routes
- Speed cushions may be effective on collector roadways and emergency routes
Advantages
- Immediately effective at reducing speeds for all passing vehicles
- Fairly inexpensive to install and maintain
- Can design for specific speed
Disadvantages
- Significant impact on emergency response times and slows transit service
- May divert traffic to other street
- Noise from braking, acceleration, and traversing
- Can feel unpleasant to traverse, especially for people with some medical conditions
Cost
$2,000 - $5,000 per speed hump or cushion
Purpose
Raised crosswalks and intersections are defined by a vertical rise in the crosswalk or intersection that is designed to slow traffic. Raised crosswalks and intersections incorporate what is known as a speed table, which is like a flat topped speed hump that is up to 22 feet wide. For a raised crosswalk, this includes pavement markings and signs to help pedestrians and drivers recognize the crosswalk. For a raised intersection, the entire interior area of the intersection is raised creating a plateau, with ramps on all the intersection approaches (or legs).
Effectiveness
- Raised crosswalks (speed tables) can be effective in reducing vehicular speeds up to 20%
- Raised intersections have limited effectiveness in speed reduction, but may be effective in promoting pedestrian safety at locations with multiple crosswalks
Advantages
- Raised crosswalks are immediately effective at reducing speeds for all passing vehicles
- Both can improve driver compliance with yielding to people in the crosswalk(s)
Disadvantages
- Significant impact on emergency response times and slows transit service
- Can impact drainage infrastructure and increase costs
- Noise from braking, acceleration, and traversing
- Can feel unpleasant to traverse, especially for people with some medical conditions
- May divert traffic to other street
Cost
- $10,000 - $20,000 for one raised crosswalk, $50,000 - $100,000 for one raised intersection
Purpose
Medians and crossing islands create a visually narrow space for cars on the road and increase the visibility of people crossing to drivers. They can include a horizontal shift in the driver's path which can cause drivers to slow down when they are parallel to or passing the median or island.
Effectiveness
Medians and crossing islands can reduce speeds up to 15%
Advantages
- Can provide space for placemaking, landscaping, and neighborhood gateways
- Can shorten the distance for people crossing the street
- Can be built to minimize impact to emergency response
Disadvantages
- Lower design speed is more effective at slowing traffic, but is also more impactful to emergency response
- Higher cost especially if landscape elements are included
Cost
$10,000 - $40,000 per median or island, depending on size and length
Purpose
Curb extensions include curb bulb-outs, chokedowns, and chicanes. A curb bulb-out is a horizontal extension of the sidewalk, located at a crosswalk, that narrows the roadway. Bulb-outs are usually built in pairs to maximize effect. A chokedown is a pair of curb extensions but is usually mid-block, intended to slow down approaching vehicles. A chicane is a series of curb extensions that create alternating curve shifts, causing a driver to travel in a path curving back and forth.
These devices create a visually narrow space on the roadway for cars or create deviations in a straight travel path to reduce speed. When used near pedestrian crossings, curb extensions can be used to prevent people from parking too close to the crosswalks and can shorten the distance of the crosswalk.
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Effectiveness
- Bulb-outs and chokedowns can reduce speeds up to 10%
- Chicanes can reduce speeds up to 20%
Advantages
- Can provide space for placemaking, landscaping, and neighborhood gateways
- Can be used to prevent people from parking close to crosswalks, providing more visibility for people using a crosswalk
- Can shorten the distance for people crossing the street
Disadvantages
- Must be clearly marked to alert drivers of the change in the roadway
- May divert traffic to another street
- Lower design speed is more effective at slowing traffic, but is also more impactful to emergency response
- Depending on design, the device can impact drainage and increase installation costs
Cost
$10,000 - $40,000 per extension
Purpose
Traffic circles are located in the middle of unsignalized intersections, and are usually used with yield signs causing the driver to slow down on approach rather than coming to a full stop. Traffic circles slow down drivers as they maneuver around the circle, whether they are traveling straight through the intersection or turning right or left. The circle, which is a raised island that may include landscaping, creates a narrowed path and causes vehicles to shift out of a straight travel pattern through the intersection. Traffic circles are different from roundabouts in that they can be installed in existing intersections and do not require lane splitter islands on the approaches (intersection legs).
Effectiveness
Traffic circles can be effective in reducing vehicular speeds between 10% - 15%, depending on design
Advantages
- Can provide space for placemaking, landscaping, and neighborhood gateways
- Effectiveness for reducing speeds depends on the diameter of the circle and radius of the turn for vehicles.
- Usually does not impact roadway drainage
Disadvantages
- Must be clearly marked to alert drivers of the change in the roadway
- Lower design speed is more effective at slowing traffic, but is also more impactful to emergency response
- Forces cars and bicycles to share space as they pass through the intersection
- Crosswalks may be impacted if they are close to the intersection
- May divert traffic to other/adjacent streets
Cost
$15,000 - $50,000 per traffic circle, depending on size, construction materials, and landscaping
Purpose
Radar speed signs display the speed of a person driving as they approach the sign, raising awareness of speed and encouraging the driver to slow down.
Effectiveness
Radar speed signs can reduce speeds up to 10% after installation, but their effectiveness decreases over time
Advantages
- Provides active feedback to people driving, which may discourage inattentive speeding
- No impacts to emergency response times
Disadvantages
- Device is not delay-inducing, and has limited effectiveness at reducing speeds
- Considerable long-term maintenance costs
Cost
One sign display can cost between $5,000 - $10,000, but they are most effective in pairs
Purpose
A diagonal diverter is a barrier that is placed through the center of an intersection, creating 2 two-way intersections out of 1 four-way intersection. A diagonal diverter causes a driver to turn either right or left depending of their direction of travel, but does not allow through traffic. Gaps or cutouts in the middle of the diverter can allow pedestrians and cyclists to continue travel through the intersection where it is restricted for vehicles.
Full and half closures restrict access at an intersection; full closures completely restrict vehicular access in both directions, whereas a half closure allows access in one direction but not the other. A half closure effectively creates a one-way street at the point of the closure.
Effectiveness
- Diagonal diverters may have limited effectiveness in reducing speeds depending on the location, but are effective in reducing traffic volumes
- Full closures may have limited effectiveness in reducing speeds depending on the location, but are effective in reducing traffic volumes
- Half closures can be as effective as curb extensions in reducing speed (that is what is usually used to restrict access), and also reduce traffic volumes
Advantages
- Can be used to discourage or restrict non-local or cut through traffic in neighborhoods
- May enhance travel for pedestrians and bicyclists by reducing traffic volumes and allowing access where cars are restricted
- Can provide space for placemaking, landscaping, and neighborhood gateways
Disadvantages
- Restricts access for all vehicles
- Significant impact on emergency response times
- Depending on design, the device can impact drainage and increase installation costs
- Diverts traffic to other/adjacent streets
Cost
$15,000 - $45,000 per diverter or closure, depending on design
Purpose
On-street parking is dedicated space for the storage of personal vehicles. Parking curbside on the street creates side friction to traffic flow, and effectively narrows the roadway width which can create a similar effect as a curb extension. On-street parking can be on either or both sides of the street, and zones can be continuous or staggered to create the feeling of a chicane. On-street parking can be parallel parking or angled parking.
Effectiveness
Depending on the usage and length of the parking zone, effectiveness may be limited or may be as effective as roadway narrowing.
Advantages
- Can be a low-cost way to traffic calm a street if parking demand results in high usage of the parking zone
- Provides the community space for the storage of private vehicles
- Can provide revenue through pricing
Disadvantages
- May not be effective if parking demand is low
- May create additional collisions with cars, bicyclists, and pedestrians as vehicles enter and exit the parking zone
- Reduces efficiency of the through lane as vehicles enter and exit the parking zone
Cost
The cost of on-street parking can be minimal if a zone is created through signs; costs increase with revenue infrastructure (pay stations), pavement markings, enforcement, and paired traffic calming (like curb bulb-outs).