lec-16,17 traffic calming

Lecture 16 and 17

TRAFFIC CALMING

Institute of Transportation Engineers – Traffic calming involves 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.

SPEED CONTROL

Speed Humps

Speed humps are rounded, raised areas placed across the roadway. They are generally 3 to 4.25 m long (in the direction of travel), making them distinct from the shorter “speed bumps” found in many parking lots, and are 75 to 100 mm high. The profile of a speed hump can be circular, parabolic, or sinusoidal. They are often tapered as they reach the curb on each end to allow unimpeded drainage.

Speed Humps are good for locations where very low speeds are desired and reasonable, and noise and fumes are not a major concern.

Advantages:

Speed Humps are relatively inexpensive

They are relatively easy for bicycles to cross if designed appropriately

They are very effective in slowing travel speeds.

Disadvantages:

They cause a “rough ride” for all drivers, and can cause severe pain for people with certain skeletal disabilities

They force large vehicles, such as emergency vehicles and those with rigid suspensions, to travel at slower speeds

They may increase noise and air pollution

They have questionable aesthetics.

Effectiveness:

For a 3.5 m hump:

o Average of 22% decrease in the 85th percentile travel speeds, or from an average of 55.0 to 45 km per hour; (from a sample of 179 sites).

o Average of 11% decrease in accidents or from an average of 2.7 to 2.4 accidents per year (from a sample of 49 sites).

For a 4.25 m hump:

o Average of 23% decrease in the 85th percentile travel speeds, or from an average of 55 to 40 km per hour (from a sample of 15 sites).

o Average of 41% decrease in accidents

Speed Tables

Speed tables are flat-topped speed humps often constructed with brick or other textured materials on the flat section. Speed tables are typically long enough for the entire wheelbase of a passenger car to rest on the flat section. Their long flat fields give speed tables higher design speeds than Speed Humps. The brick or other textured materials improve the appearance of speed tables, draw attention to them, and may enhance safety and speed-reduction.

Speed tables are good for locations where low speeds are desired but a somewhat smooth ride is needed for larger vehicles.

Advantages:

They are smoother on large vehicles (such as fire trucks) than Speed Humps

They are effective in reducing speeds, though not to the extent of Speed Humps

Disadvantages:

They have questionable aesthetics, if no textured materials are used;

Textured materials, if used, can be expensive; and

They may increase noise and air pollution.

Raised Crosswalks

Raised crosswalks are Speed Tables outfitted with crosswalk markings and signage to channelize pedestrian crossings, providing pedestrians with a level street crossing. Also, by raising the level of the crossing, pedestrians are more visible to approaching motorists.

Raised crosswalks are good for locations where pedestrian crossings occur at haphazard locations and vehicle speeds are excessive.

Advantages:

Raised Crosswalks improve safety for both pedestrians and vehicles

If designed well, they can have positive aesthetic value

They are effective in reducing speeds, though not to the extent of Speed Humps

Disadvantages:

Textured materials, if used, can be expensive

Their impacts on drainage needs to be considered

They may increase noise and air pollution

Raised Intersections

Beaverton-This raised crosswalk uses asphalt Eugene- This raised crosswalk uses a

and highly-visible paint. unique pattern of concrete and brick.

Raised intersections are flat raised areas covering an entire intersection, with ramps on all approaches and often with brick or other textured materials on the flat section. They usually raise to the level of the sidewalk, or slightly below to provide a “lip” that is detectable by the visually impaired. By modifying the level of the intersection, the crosswalks are more readily perceived by motorists to be “pedestrian territory”.

Raised intersections are good for intersections with substantial pedestrian activity, and areas where other traffic calming measures would be unacceptable because they take away scarce parking spaces.

Advantages:

Raised Intersections improve safety for both pedestrians and vehicles


They can calm two streets at once

Disadvantages:

They tend to be expensive, varying by materials used

Their impact to drainage needs to be considered

They are less effective in reducing speeds than Speed Humps, Speed Tables, or Raised Crosswalks

Textured Pavement

Textured and colored pavement includes the use of stamped pavement or alternate paving materials to create an uneven surface for vehicles to traverse. They me be used to emphasize either an entire intersection or a pedestrian crossing, and are sometimes used along entire street blocks.

Textured pavements are good for “main street” areas where there is substantial pedestrian activity and noise is not a major concern.

Advantages:

Textured Pavements can reduce vehicle speeds over an extended length


Placed at an intersection, they can calm two streets at once

Disadvantages:

They are generally expensive, varying by materials used

If used on a crosswalk, they can make crossings more difficult for wheelchair users and the visually impaired

Traffic Circles

Traffic circles are raised islands, placed in intersections, around which traffic circulates. They are good for calming intersections, especially within neighborhoods, where large vehicle traffic is not a major concern but speeds, volumes, and safety are problems.

Advantages:

Traffic Circles are very effective in moderating speeds and improving safety


Placed at an intersection, they can calm two streets at once

Disadvantages:

They are difficult for large vehicles (such as fire trucks) to circumnavigate

They must be designed so that the circulating lane does not encroach on the crosswalks

They may require the elimination of some on-street parking

Landscaping must be maintained, either by the residents or by the municipality

Roundabouts

Roundabouts require traffic to circulate counterclockwise around a center island. Unlike Traffic Circles, roundabouts are used on higher volume streets to allocate right-of-way between competing movements.

Good for:

Locations with a history of accidents;

Intersections where queues need to be minimized

Intersections with irregular approach geometry

Providing inexpensive-to-operate traffic control as an alternative to a traffic signal

Handling a high proportion of U-turns

Locations with abundant right-of-way

Advantages:

Roundabouts can moderate traffic speeds on an arterial

They are generally aesthetically pleasing if well landscaped

They enhanced safety compared to traffic signals

They can minimize queuing at the approaches to the intersection

They are less expensive to operate than traffic signals

Disadvantages:

They may be difficult for large vehicles (such as fire trucks) to circumnavigate

They must be designed so that the circulating lane does not encroach on the crosswalks


Landscaping must be maintained, either by the residents or by the municipality

Chicanes

Chicanes are curb extensions that alternate from one side of the street to the other, forming S-shaped curves. Chicanes can also be created by alternating on-street parking, either diagonal or parallel, between one side of the street and the other. Each parking bay can be created either by restriping the roadway or by installing raised, landscaping islands at the ends of each parking bay.

Good for locations where speeds are a problem but noise associated with Speed Humps and related measures would be unacceptable.

Advantages:

Chicanes discourage high speeds by forcing horizontal deflection

They are easily negotiable by large vehicles (such as fire trucks) except under heavy traffic conditions

Disadvantages:

They must be designed carefully to discourage drivers from deviating out of the appropriate lane

Curb realignment and landscaping can be costly, especially if there are drainage issues


Re-aligned Intersections

Realigned intersections are changes in alignment that convert T-intersections with straight approaches into curving streets that meet at right-angles. A former “straight-through” movement along the top of the T becomes a turning movement. While not commonly used, they are one of the few traffic calming measures for T-intersections, because the straight top of the T makes deflection difficult to achieve, as needed for Traffic Circles. They are good for T-intersections.

Advantages:

Realigned Intersections can be effective in reducing speeds and improving safety at a T-intersection that is commonly ignored by motorists

Disadvantages:

The curb realignment can be costly

They may require some additional right-of-way to cut the corner

Neckdowns

Neckdowns are curb extensions at intersections that reduce the roadway width from curb to curb. They “pedestrianize” intersections by shortening crossing distances for pedestrians and drawing attention to pedestrians via raised peninsulas. They also tighten the curb radii at the corners, reducing the speeds of turning vehicles.

They are good for intersections with substantial pedestrian activity and areas where vertical traffic calming measures would be unacceptable because of noise considerations.

Advantages:

Neckdowns improves pedestrian circulation and space

Through and left-turn movements are easily negotiable by large vehicles

They create protected on-street parking bays

They reduce speeds, especially for right-turning vehicles

Disadvantages:

Effectiveness is limited by the absence of vertical or horizontal deflection

They may slow right-turning emergency vehicles

They may require the elimination of some on-street parking near the intersection

They may require bicyclists to briefly merge with vehicular traffic

Center-Island Narrowings

A center island narrowing is a raised island located along the centerline of a street that narrow the travel lanes at that location. Center island narrowings are often landscaped to provide a visual amenity. Placed at the entrance to a neighborhood, and often combined with textured pavement, they are often called “gateway islands.” Fitted with a gap to allow pedestrians to walks through at a crosswalk, they are often called “pedestrian refuges.”

Center Island Narrowings are good for entrances to residential areas, and wide streets where pedestrians need to cross.

Advantages:

Center Island Narrowings increase pedestrian safety


They reduce traffic volumes

Disadvantages:

Their speed-reduction effect is somewhat limited by the absence of any vertical or horizontal deflection

They may require elimination of some on-street parking

Chokers

Chokers are curb extensions at midblock locations that narrow a street by widening the sidewalk or planting strip. If marked as crosswalks, they are also known as safe crosses. Two-lane chokers leave the street cross section with two lanes that are narrower than the normal cross section. One-lane chokers narrow the width to allow travel in only one direction at a time, operating similarly to one-lane bridges. They are good for areas with substantial speed problems and no on-street parking shortage.

Advantages:

Chokers are easily negotiable by large vehicles (such as fire trucks)


They reduce both speeds and volumes

Disadvantages:

Their effect on vehicle speeds is limited by the absence of any vertical or horizontal deflection

They may require bicyclists to briefly merge with vehicular traffic


VOLUME CONTROL

Full Closures

Full street closures are barriers placed across a street to completed close the street to through-traffic, usually leaving only sidewalks open. They are good for locations with extreme traffic volume problems and several other measures have been unsuccessful.

Advantages:

Full Closures are able to maintain pedestrian and bicycle access

They are very effective in reducing traffic volume

Disadvantages:

They require legal procedures for street closures (in California – varies by state)

They cause circuitous routes for local residents and emergency services

They may be expensive

They may limit access to businesses

Half Closures

Half closures are barriers that block travel in one direction for a short distance on otherwise two-way streets. They are good for locations with extreme traffic volume problems and non-restrictive measures have been unsuccessful.

Advantages:

Half Closures are able to maintain two-way bicycle access

They are effective in reducing traffic volumes

Disadvantages:

They causes circuitous routes for local residents and emergency services

They may limit access to businesses

Depending on the design, drivers may be able to circumvent the barrier

Diagonal Diverters

Diagonal diverters are barriers placed diagonally across an intersection, blocking through movements and creating two separate, L-shaped streets. Like half closures, diagonal diverters are often staggered to create circuitous routes through the neighborhood as a whole, discouraging non-local traffic while maintaining access for local residents. They are good for inner-neighborhood locations with non-local traffic volume problems.

Advantages:

Diagonal Diverters do not require a closure per se, only a redirection of existing streets

They are able to maintain full pedestrian and bicycle access

They reduce traffic volumes

Disadvantages:

They cause circuitous routes for local residents and emergency services

They may be expensive

They may require reconstruction of corner curbs

Median Barriers

Median barriers are islands located along the centerline of a street and continuing through an intersection so as to block through movement at a cross street.

They are good for:

Local street connections to main streets where through traffic along the continuing local street is a problem

Main streets where left-turns to and/or from the side street are unsafe

Advantages:

Median Barriers can improve safety at an intersection of a local street and a major street by prohibiting dangerous turning movements

They can reduce traffic volumes on a cut-through route that crosses a major street

Disadvantages:

They require available street width on the major street

They limit turns to and from the side street for local residents and emergency services

Effectiveness

Speed Impacts of Traffic Calming Measures

(standard deviations in parentheses)

Sample Size

85th Percenile Speed Afterward

Average Change in 85th Percentile Speed

Average % Change

12' Speed Hump 179 27.4 mph (4.0 mph)

-7.6 mph (3.5mph)

-22% (9%)

14' Speed Hump 15 25.6 (2.1)

-7.7 (2.1 mph)

-23 (6)

22' Speed Table 58 30.1 (2.7)

-6.6 (3.2)

-18 (8)

Longer Table (>22') 10 31.6 (2.8)

-3.2 (2.4)

-9 (7)

Raised Intersection 3 34.3 (6.0)

-0.3 (3.8)

-1 (10)

Traffic Circle 45 30.3 (4.3)

-3.9 (3.2)

-11 (10)

Narrowing 7 32.3 (2.8)

-2.6 (5.5)

-7 (22)

Choker 5 28.6 (3.1)

-2.6 (1.3)

-14 (4)

Half Closure 16 26.3 (5.2)

-6.0 (3.6)

-19 (11)

Diagonal Diverter 7 27.9 (5.2)

-1.4 (4.7)

0 (17)

Note: speeds are measured at midpoints between measures

Volume Impacts of Traffic Calming Measures

(standard deviations in parentheses)

Sample Size Average Change in Volume Average % Change

Choker 5 -392 vehicles per day (384 vehicles per day)

-20% (19%)

Full Closure 19 -671 vehicles per day (786 vehicles per day)

-44% (36%)

Half Closure 53 -1611 vehicles per day (2444 vehicles per day)

-42% (41%)

Diagonal Diverter 27 -501 vehicles per day (622 vehicles per day)

-35% (46%)

Other Volume Control 10 -1167 vehicles per day (1781 vehicles per day)

-31% (36%)

Safety Impacts of Traffic Calming Measures

(U.S. Experience)

# of Observations

Average # of Collisions Without Measure

Average # of Collisions With Measure

% Change in Collisions

12' Speed Hump 49 2.7 2.4 -11%

14' Speed Hump 5 4.4 2.6 -41%

22' Speed Table 8 6.7 3.7 -45%

Traffic Circle (w/o Seattle)

17 5.9 4.2 -29%

Traffic Circle (w/Seattle)

130 2.2 0.6 -73%

All Measures

w/o Adjustments 192 2.6 1.3 -50%*

w/Adjustments 42 3.8 3.0 -21%**

* Significant at 0.001 probablity level ** Significant at 0.04 probability level

lec-16,17 traffic calming

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