Speed Bumps And Speed Humps

Continued from…Reducing street area where motor traffic is given priority.

Slow Point Examples: Medians

Medians

Medians are islands located along the roadway centerline, separating opposing directions of traffic
movement. They can be either raised or flush with the level of the roadway surface. They can be expressed as painted pavement markings, raised concrete platforms, landscaped areas, or any of a variety of other design forms. Medians can provide special facilities to accommodate pedestrians and
bicyclists, especially at crossings of major roadways.

Design Considerations:
• Medians are most valuable on major, multi-lane roads that present safety problems for bicyclists
and pedestrians wishing to cross. The minimum central refuge width for safe use by those with
wheelchairs, bicycles, baby buggies, etc. is 1.6 meters (2 meters is desirable).
• Where medians are used as pedestrian and bicyclist refuges, internally illuminated bollards are suggested on the medians to facilitate quick and easy identification.
• Used in isolation, roadway medians do not have a significant impact in reducing vehicle speeds. For the purpose of slowing traffic, medians are generally used in conjunction with other devices, such as curb extensions or roadway lane narrowing.

Several caveats apply:
• To achieve meaningful speed reductions, the travel lane width reduction must be substantial and visually obvious. The slowing, however, is temporary; as soon as the roadway widens again, traffic resumes its normal speed.
• Bicyclists have been put at risk of being squeezed where insufficient room has been left between a central median and the adjacent curb. Experience shows that most drivers are unlikely to hold back in such instances to let bicyclists go through first. This threat is particularly serious on roads with high proportions of heavy vehicles.
• The contradiction between the need to reduce the roadway width sufficiently to lower motorist speeds, while at the same time leaving  enough room for bicyclists to ride safely, must be addressed. This may be achieved by reducing the roadway width to the minimum necessary for a bicyclist and a motorist to pass safely (i.e., 3.5 meters).

There are three suggestions:
• Introducing color or texture changes to the road surface material around the refuge area reminds motorist that a speed reduction is intended.
• White striping gives a visual impression that vehicles are confined to a narrower roadway than that created by the physical obstruction — adjacent areas exist that vehicles can run over, but these are not generally apparent to approaching drivers.
• In some cases, provide an alternate, cut-through route for the bicyclists.

Next… Slow Point Examples:Curb Extensions

Continued from…Intersection Humps/Raised Intersections:

2. Reducing street areas where motor traffic is given priority. This category of traffic-calming includes all those that reduce the area of the street designated exclusively for motor vehicle travel. “Reclaimed” space is typically used for landscaping, pedestrian amenities, and parking.

Discussed here are:
• Slow points.
• Medians.
• Curb extensions.
• Corner radius treatment.
• Narrow traffic lanes.

Slow Points (neck-downs, traffic throttles, pinch points) - Slow points narrow a two-way road over a short
distance, forcing motorists to slow and, in some cases, to merge into a single lane. Sometimes these are used in conjunction with a speed table and  coincident with a pedestrian crossing. The following are advantages and disadvantages of both one-lane and two-lane slow points:

(1) One-lane slow point.
One-lane slow points restrict traffic flow to one lane. This lane must accommodate motor traffic in both travel directions. Passage through the slow point can be either straight through or angled.
Advantages:
• Vehicle speed reduced.
• Most effective when used in a series.
• Imposes minimal inconvenience to local traffic.
• Pedestrians have a reduced crossing distance, greater safety.
Disadvantages:
• Reduced sight distances if landscaping is not low and trimmed.
• Contrary to driver expectations of unobstructed flow.
• Can be hazardous for drivers and bicyclists if not designed and maintained properly.
• Opposing drivers arriving simultaneously can create confrontation.

(2) Two-lane slow point.
Two-lane slow points narrow the roadway while providing one travel lane in each direction.
Advantages:
• Only a minor inconvenience to drivers.
• Regulates parking and protects parked vehicles as the narrowing can help stop illegal parking.
• Pedestrian crossing distances reduced.
• Space for landscaping provided.
Disadvantages:
• Not very effective in slowing vehicles or diverting through traffic.
• Only partially effective as a visual obstruction.

Design Considerations:
• Where slow points have been used in isolation as speed control measures, bicyclists have felt squeezed as motorists attempt to overtake them at the narrowing. Not all bicyclists have the confidence to position themselves in the middle of the road to prevent overtaking on the approach to and passage through the narrow area.
• To reduce the risk of bicyclists’ being squeezed, slow points should generally be used in conjunction
with other speed control devices such as speed tables at the narrowing. Slower moving drivers will be more inclined to allow bicyclists through before trying to pass. Where bicycle flows are high, consideration should be given to  a separate right-of-way for bicyclists past the narrow area.
• A textured surface such as brick or pavers may be used to emphasize pedestrian crossing movement. Substituting this for the normal roadway surface material may also help to impress upon motorists that lower speeds are intended.
• Such measures should not confuse pedestrians with respect to the boundary of the roadway area over which due care should still be taken. In particular, where a road is raised to the level of the adjacent sidewalk, this can cause problems for those with poor sight. However, a tactile strip may help blind people in distinguishing between the roadway and the sidewalk; similarly, a color variation will aid those who are partially sighted.
• Slow points can be used to discourage use of the street by large vehicles. They can, however, be barriers to fire trucks and other emergency vehicles. Some designs permit access by emergency vehicles by means of lockable posts or ramped islands.
• Slow points can enhance the appearance of the street. For example, landscaped islands can be installed, intruding into the roadway to form a narrow “gate” through which drivers must pass. Landscaping enhances the neighborhood’s sense of nature and provides a visual break in views along the street.
• Slow points are generally only sanctioned where traffic flows are less then 4,000 to 5,000 vehicles per day. Above this level, considerable delays will occur during peak periods.
• Clear signing should indicate traffic flow priorities.

Next… Slow Point Examples

Intersection Humps/Raised Intersections
continued from Raised Crosswalks

Intersection humps raise the roadway at the intersection, forming a type of “plateau” across the intersection, with a ramp on each approach.  The plateau is at curb level and can be enhanced through the use of distinctive surfacing such as pavement coloring, brickwork, or other pavements. In some cases, the distinction between roadway and sidewalk surfaces is blurred. If this is done, physical obstructions such as bollards or planters should be considered, restricting the area to which motor vehicles have access.

Design Considerations:
• Ramps should not exceed a maximum gradient of 16 percent.
• Raised and/or textured surfaces can be used to alert drivers to the need for particular care.
• Distinctive surfacing helps reinforce the concept of a “calmed” area and thus plays a part in reducing vehicle speeds.
• Distinctive surfacing materials should be skid-resistant, particularly on inclines.
• Ramps should be clearly marked to enable bicyclists to identify and anticipate them, particularly under conditions of poor visibility.
• Care must be taken so the visually impaired have adequate cues to identify the roadway’s location (e.g., tactile strips). Color contrasts will aid those who are partially sighted.

Next… Traffic Calming - Reducing Street Area Where Motor Vehicle Traffic is a Priority.

Raised Crosswalks

(Continued from Bumps, Humps, & Other Raised Pavement Areas)

Raised crosswalks are essentially broad, flat-topped speed humps that coincide with pedestrian crosswalks at street intersections. The crosswalks are raised above the level of the roadway to slow traffic, enhance crosswalk visibility, and make the crossing easier for pedestrians who may have difficulty stepping up and down curbs.

Raised Crosswalks Design Considerations:
• Can be constructed of brick, concrete block, colored asphalt or cement, with ramps striped for
better visibility.
• Raised crosswalks are applicable:
(1) On roadways with vehicular speeds perceived as being incompatible with the adjacent residential                       land uses.
(2) Where there is a significant number of pedestrian crossings.
(3) In conjunction with other traffic-calming devices, particularly entry treatments.
(4) On two-lane or fewer residential streets classified as either “local streets” or neighborhood
collector streets.”
(5) On roadways with 85th percentile speeds less than 45 mph.

Next… Intersection Humps/Raised Intersections

When any new traffic management approach is introduced, issues, concerns, and questions are bound
to arise. Design decisions related to traffic can have far-reaching consequences. Lives, economic well-being, and urban livability are directly affected.

Professional engineers, planners, government, and the public all are aware of and sensitive to proposals
for changes in the traffic environment. Roadway congestion, air quality, traffic safety, street crimes,
and the high cost of new improvements are among the most-widely debated issues in America today. New design ideas are, and should be, subjected to rigorous testing and evaluation before being accepted as
part of the standard engineering and transportation planning tool kit. Traffic calming is not a panacea for
urban transportation woes, but it can have significant benefits in many situations.

In considering the application of traffic-calming techniques, what specific issues are likely to arise? The discussion on the following pages focuses on traffic-calming issues.

1. Traffic safety.

The Issue: Encouraging people to walk, play, and bicycle in and next to the streets is just asking for trouble. They will have a false sense of security and accidents will increase. They will develop bad habits that may increase their when they leave the neighborhood.

Comment: Traffic-calming measures have been implemented in many European cities. In the Netherlands and Germany, extensive research has been conducted to evaluate the safety and impact of traffic-calming techniques and devices.
2. Impact on traffic volumes, distribution, and operations.

The Issue: Traffic calming will never work on anything except very low-volume residential streets. It will substantially reduce the amount of traffic that a street can handle efficiently and this is counterproductive.
We need to move vehicles, not restrict them.  Furthermore, if we slow traffic on one streets, the traffic will simply be diverted to another street. The net result will be increased congestion and more problems overall.

Comment: A 5-year German Federal Government evaluation of traffic calming and follow-up research
found:
• Little change in overall traffic volumes.
• Reduction in average vehicle speeds by almost 50 percent.
• Average increase in motorist trip time of only 33 seconds.

3. Lack of proven design standards.

The Issue: There are no uniform, accepted, and legally defensible standards to follow. If we want to try traffic calming, where can we get specific information about design?
Comment: Many U.S. cities are now developing and testing design guidelines for traffic-calming improvements. Although uniform, national standards have yet to evolve, valuable experience is being gained. The list of references in this blog provides a starting point for further exploration of
specific design approaches.
4. Liability.
The Issue: These traffic-calming ideas may be accepted in Europe, but they haven’t really been tried here. Are we opening the door to all kinds of legal problems if somebody crashes on a traffic circle or a speed table and sues us?

Comment: When considering the use of any new design approach, concerns about liability can be addressed somewhat through performance of “due diligence” on the part of the engineer, planner, or other professionals involved in the design. Research into the experiences of other U.S. cities, European standards, and evaluation studies should be thorough and followed up with a first-hand look if possible. Construction of a pilot project or other testing of proposed designs can benefit, as can ongoing and systematic valuation of the improvements once installed.

5. Emergency and service vehicle access.

The Issue: Construction of speed bumps, neckdowns, medians, and traffic circles will increase response times for emergency vehicles and may restrict access for garbage trucks, delivery vans, and other large vehicles.

Comment: Studies in Berkley and Palo Alto, CA, show that traffic management measures (e.g., traffic diverters, bicycle boulevards) have not impaired police or fire emergency response times. • The Seattle Engineering Department works closely with its Fire Department to design and field-test traffic circles on a site-specific basis to ensure good emergency access.
6. Impacts on bicycling.

The Issue: Pavement texturing, speed tables, wider sidewalks, “bulb-outs” at corners and similar
improvements may make things better for pedestrians, but may have a negative impact on bicycling.
Emergency vehicle access should always be considered when incorporating traffic-calming
measures.

Comment: A 5-year German Federal Government evaluation of traffic calming and follow-up research found doubling of bicycle use over a 4-year period.

• Implementation of traffic management strategies in the downtownarea of the Dutch City of Groningen contributed to a substantial increase in bicycling and walking. Bicycle use is now well over 50 percent of all trips.
• Studies of traffic-calming areas in Japan show increases in both bicycle and pedestrian traffic volumes along most routes.