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V. TECHNIQUES

  This section provides a "tool box" of traffic management and traffic calming techniques that are available for consideration and use.

Choose the Right Tools
Any job is made easier by using the right tools. In the management and calming of traffic, strategies and solutions are based on two fundamental ideas:

Experience from other communities around the country has indicated that it is important to use a variety of traffic management techniques, and that the tools selected should be tailored to fit each situation.

Aesthetics
Aesthetics should be considered in the process of developing traffic management solutions. To be successful, traffic management and calming measures should achieve a balance between aesthetics and the objective of calming traffic as no program will succeed if it is not based on community support. It is the intent of this program to develop solutions where safety, speed control and aesthetics are mutually supportive.

Selection of Tools
Almost all traffic management techniques (tools) have some effect on both traffic volume and speed. However, it is evident that some tools used to calm traffic can have a greater or lesser effect than others in controlling either volume or speed. Also, some tools are more effective and desirable on local neighborhood streets and less desirable for complex traffic management applications on collectors, minor arterials, or emergency response routes.

Emphasis in this program will be given to the selection of tools which are self-enforcing - that is, those which are physical traffic management measures. These tools are designed to affect driver perception of the street or neighborhood and are designed to directly influence motorist behavior. Unlike traditional methods of traffic management, such as the use of stop signs or speed limit signs, physical traffic calming measures rely on physically modifying driver behavior in a neighborhood.

Diverting Traffic Intentionally to Other Streets
In selecting tools to be used on streets it is important to limit the potential detrimental effects of diverting excessive volumes of traffic from one local street to another, or from collector or minor arterial streets to local streets. The potential for the diversion of traffic is considered in the selection of tools for all neighborhood traffic management projects.

Since neighborhood collectors, and in particular, minor arterials, are meant to serve as through streets, traffic management projects for these situations are not primarily designed to decrease traffic volumes. Instead, emphasis is on reducing traffic speeds, enhancing pedestrian usage, and associated concerns.

Choosing Other Minor Arterial Streets
For complex traffic management projects, while it is not expected that the primary result will be to divert traffic away from the project street, some decrease in traffic volumes may occur as a result of the project. For example, some drivers may prefer not to use the traffic calmed street after the devices are installed. One outcome may be that drivers may choose to use other arterial streets instead. This is an appropriate use of these arterials and represents an objective of the program since they are intended to serve through traffic.
 
Techniques for Local Street Projects

Speed Control Measures
Speed control measures are of three types: vertical measures, which use the forces of vertical acceleration to discourage speeding; horizontal measures, which use the forces of lateral acceleration to discourage speeding; and narrowings, which use a psycho- perceptive sense of enclosure to discourage speeding. Tables 4, 5, and 6 below summarize the measures and their effects.

Vertical Measures
speed hump Speed humps are rounded raised areas placed across the road (Figure 1).The standard speed hump used in the City of Santa Fe is 14 feet long and 3 inches high. Its design speed (speed at which the 85th percentile is estimated to be) is 18 to 23 mph. Usually these humps are constructed with a taper on each side to provide for drainage. In some locations it may be desirable to provide a space wide enough to accommodate bicycles but this may also encourage motorists to cross the hump with one wheel in. the gutter and the other on the hump. Speed humps may be located on streets where gutters exist, but considerations must be given to pedestrians if sidewalks are not in place. If no curb and gutter is in place, then devices such as bollards may be placed on each side of the humps to discourage motorists from driving on the shoulders.

speed table Speed tables are flat-topped speed humps and can be constructed with brick or other textured materials on the flat section (Figure 2). These devices have the advantage of possibly being used to provide a pedestrian crossing and can be marked and placed at intersections or other pedestrian crossing locations. In this case they are called raised crosswalks. These devices are long enough for the entire wheelbase of a passenger car to rest on the top. The use of brick or other textured materials improves the appearance and tends to draw attention to them and may enhance safety and speed reductions. The standard speed table used in the city of Santa Fe is 22 feet long and 3 inches high. Its design speed is 25 to 30 mph. In effect the device is less jarring than a speed hump and reduces higher operating speeds with out affecting the average driver. There is an alternate version of this device that may be used to calm traffic at an entire intersection called a raised intersection (Figure 3).

raised intersection

Table 4 - Traffic Calming Tools for Local Street Projects
Speed Control Measures - Vertical
Traffic Management Device Speed Reduction Volume reduction Noise and Pollution Emergency Vehicle Access Maintenance Access Restrictions Safety
Speed Humps Yes Traffic diversion possible Increase Some problems Street cleaning difficult None Unclear
Speed Tables Yes Possible Some Increase Acceptable Street cleaning difficult None Improved Ped Safety
Raised Intersection Yes Possible Small Increase Acceptable Street cleaning difficult None Improved Ped Safety

Horizontal Measures
Horizontal measures achieve speed reduction by forcing drivers to drive around a device or to change direction, and by visually blocking long views of the road ahead.

Traffic circles are raised islands located at intersections around which traffic circulates (Figure 4). They are usually, though not always, circular in shape. A circle prevents drivers from speeding through an intersection by eliminating the straight through movement and forcing drivers to slow down to yield. Drivers must first turn to the right, then to the left as they pass the circle, and then back to the right again after clearing the circle. Large trucks can be accommodated by design modifications to the circle. Traffic circles are very effective in controlling vehicle speeds at an intersection - generally the design speed is 15 to 20 mph for passenger cars. At mid-block locations vehicles can speed up. The safety record is excellent as they generally reduce intersection-related accidents by up to 90 percent.

Chicanes are curb extensions that alternate from one side of the street to the other, forming S-shaped curves (Figures 5 and 6). They can prevent speeding by causing a change in direction of drivers and are speed control devices that are used in mid-block areas.

Lateral Shifts are curb extensions on otherwise straight streets that cause travel lanes to bend one way and then back to the other way to the original direction of travel (Figure 7). These devices can be used in conjunction with the creation of parking areas on streets. The design speed of this device is based on how much the street is bent and reductions in speeds can be quite significant.

traffic circle chicane
chicane with center island lateral shift with center island

Table 5 - Traffic Calming for Local Street Projects
Speed Control Measures - Horizontal
Traffic Management Device Speed Reduction Volume Reduction Noise and Pollution Emergency Vehicle Access Maintenance Access Restrictions Safety
Traffic Circles Yes, within Intersection Possible Some increase Some Limitations Vandalism None Improved
Chicanes Possible Possible Possible pollution increase Possible problems None None Possible improvement
Chicanes w/center Island Yes Possible Possible pollution increase Possible problems None None Possible improvement
Lateral Shifts w/center Islands Yes Possible Small increase Possible problems None None Possible improvement

Narrowings
Narrowing is usually accomplished by placing vertical elements in the roadway, and is intended to draw attention to a constriction in the road and to have a visual affect on drivers. For example, the roadway could be narrowed, and plantings or street furniture could be placed in the right-of-way. Neckdowns (bulbouts) are curb extensions at intersections that reduce the roadway width (Figure 8). If coupled with crosswalks they are called safe crosses. The main objective of this device is to "pedestrian-ize" intersections by shortening the crossing distance and to reduce the speed of turning vehicles.
bulbouts center island narrowings

Center island narrowings are raised islands located in the center of a street that narrow the travel lanes at that location. They can be attractively landscaped and are successful in providing a visual amenity and a neighborhood identity. If used at the entrance to a neighborhood and supplemented with textured pavement and monument signs they are called gateways (Figure 10). Centered islands have been effectively used on curves where speeding is a concern. They eliminate the possibility of drivers swinging wide to speed though curves. Uke other narrowings, these devices can help to pedestrainize streets as they provide a refuge area for pedestrians~in the center of the street.

Chokers are curb extensions at mid-block that narrow a street by widening the sidewalk or planting strip (Figures 11 and 12). Chokers should generally be placed to provide a minimum of two traffic lanes, since if only one lane is provided opposing traffic will compete for space or right of way.
gateways chokers
chokers with center islands

Table 6 - Traffic Calming Tools for Local Street Projects
Speed Control Measures - Narrowings
Traffic Management Device Speed Reduction Volume Reduction Noise and Pollution Emergency Vehicle Access Maintenance Access Restrictions Safety
Bulbouts Yes, turning speeds reduced No effect No effect Some limitation None None Improved Ped Safety
Center Island Narrowings Yes, possible on curves or turns Not likely No effect Some limitation None None Improved Ped Safety
Gateways Possible Mixed Results No effect Possible problems None None Possible Improvement
Chokers Possible, minor Possible No effect No effect Trucks hit curbs None Improved Ped Safety



Volume Control Measures
Table 7, entitled Traffic Calming Tools of Local Street Projects, summarizes the measures and their effects.

Full street closures are barriers placed across a street to close the street completely to through traffic, usually leaving sidewalks or bicycle paths open (Figures 13 and 14). The barriers may consist of landscaped islands, walls, side-by-side bollards, or any other obstructions that leave an opening smaller than the width of a passenger car. These devices are a possible solution for cut-through traffic but are very controversial. The main concerns are the effect of the closures on emergency response, street network connectivity and that parallel streets may receive diverted traffic. This device is considered as a last resort if all other devices have failed.
full street closure at mid-block full street closure at intersection

Half closures are barriers that block travel in one direction for a short distance on otherwise two-way streets. If used at an intersedion (two are placed together) the result is a semi-diverter. These devices are typically used in gridded streets to make traffic more circuitous rather than direct.

Diagonal divertors are barriers placed diagonally across an intersection, blocking through movement. These are designed to create circuitous routes through neighborhoods. As with the full street closure, there are concerns with the effect of the closures on emergency response, street network connectivity and traffic diversion.
half street closure, one side of intersection half street closure, both sides of intersection with bicycle access
diagonal diverter diagonal diverter with bicycle access

Median barriers are raised islands located in the center of a street and continuing through an intersection so as to block through movements and left-hand turns at a cross street (Figures 19 and 20). Forced turn islands are raised islands that block certain movements on approaches to an intersection (Figure 21).
median barrier median barrier with bicycle 
        and padestrian access improvements
forced turn islands


These volume control devices should be used with caution and only after the situation has have been thoroughly studied. In almost all cases, it is necessary to test these devices on a temporary basis in the field before final implementation. This is because of concerns with traffic being diverted from streets that are calmed to parallel streets that are not.

Table 7- Traffic Calming Tools for Local Street Projects
Volume Control Measures
Traffic Management Device Speed Reduction Volume Reduction Noise and Pollution Emergency Vehicle Access Maintenance Access Restrictions Safety
Full Street Closures Possible Yes Decrease Possible problems Possible problems, vandalism All non-local traffic diverted to other streets Possible, may shift accidents
Half Street Closures Possible Yes Small increase No effect None One direction, left/ right turn only Improved ped safety, may shift accidents
Diagonal Diverters Likely Yes Decrease Possible problems Vandalism Through traffic eliminated Possible improvement
Median Barriers None Yes Decrease Minor limitation None Right turns only Improved
Forced Turn Islands Possibly Yes Decrease Possible limitation Vandalism Right turns only Improved


 
Techniques for Complex Traffic Management Projects

The traffic management tools described for Local Street Projects were, for the most part, intended for local neighborhood streets that have primarily residential frontage and that are not designated as Emergency Response Routes. This section addresses all other streets that might be eligible for consideration as a Neighborhood Traffic Management Project. This includes projects that might deal with complex traffic management issues, such as treatment of a system of streets, emergency response routes, areas with complex access and pedestrian issues, or instances where significant diversion of traffic may occur.

Neighborhood collectors are intended to distribute traffic between more principal streets and local streets in the neighborhood. In other words, they are commonly called "through streets" - at least for a limited distance of generally at least V2 mile to no more than 1 mile in length. The streets may also serve multiple purposes, including carrying emergency response routes, buses, bicycles, and possibly trucks.

Minor arterials are meant to serve as through streets, and to interconnect with and augment the principal arterial system. Trips on these streets may be longer in nature than collectors, but are shorter than what should normally be carried on principal arterials.

Whether by design or as a result of growth, minor arterials and some collectors may carry a large number of vehicles through residential areas. When this occurs, the result can be a call by residents for traffic management or traffic calming to restore or improve their quality of life and mitigate the effects of unreasonable through traffic. These situations are more difficult to resolve and are typically controversial in nature.

Traffic calming of residential collectors and minor arterials has been relatively common In Europe for over a decade, but not very common in the United States. The tools discussed in this section are primarily drawn from European experiences and are based on the goal of enhancing the livability of neighborhoods with the primary objective of promoting pleasant conditions for users of neighborhoods, especially pedestrians. As indicated above, collectors and minor arterials do serve a function of carrying varying amounts of traffic. Based again on the European experience, the NTMP program objective of reducing the average speeds of traffic on neighborhood streets will be of major importance.

As indicated above, devices selected cannot intentionally divert traffic away from project streets into inappropriate areas. For example, a complex street project should not divert traffic on to an adjacent local neighborhood street, placing an unacceptable burden on that street. For complex traffic management projects, while it is not the expected that the primary result will be to intentionally divert traffic away from the project street, some decrease in traffic volumes will likely occur as a result of the project. For example, some drivers may prefer not to use the traffic calmed street after the devices are installed since higher speeds will not be possible.

Selection of Appropriate Measures
Selection of traffic calming measures for complex street projects will be based on the type of street and its functional class. As indicated above, the consideration of functional class in selecting measures attempts to strike a balance between mobility and other objectives such as compatibility with land use, and pedestrian/bicycle friendliness. For sub collector streets, mobility (the design speed of the street) may not be as crucial as other objectives. For minor arterials, with multiple lanes in each direction, the reverse is true. For streets, which are designated emergency response routes and transit routes, providing for mobility is a concern and needs to be addressed.

General Considerations
The NTMP program objective of reducing the average speeds of traffic on neighborhood streets will also be of major importance on collectors or minor arterials treated as a complex project. The primary measure used to determine the type of control and the spacing of devices will be the selection of an appropriate design speed. Table 8, entitled Santa Fe Speed/Priority of Classification of Roads for Traffic Calming, illustrates the design speed concept. For purposes of traffic calming, the design speed should be the same as the posted speed limit as this is what is appropriate based on the conditions of the street and environment.

Table 8 - Santa Fe Speed/Priority of Classification of Roads for Traffic Calming
Class Type of Classification Design Speed
Mixed Priority - Priority is shared between living and traffic functions collector 25 mph
Minor Arterial 25 mph
Traffic Street - Major access and through routes, traffic function takes priority but vulnerable road users must be protected collector 30 mph
Minor Arterial 30 mph

Selection of Measures and Spacing
Once the design speed and the class of road are selected, traffic calming measures (sometimes called slow points) and the spacing of devices can be selected. Table 9 shows guidelines for the spacing of slow points.

Table 9 - Spacing of Traffic Calming Devices (Slow Points) for Different Speeds
Desired Speed (mph) Distance between Slow Points
30 450
25 300
20 225

Examples of traffic calming measures are shown below. The measures used are primarily related to four categories of tools which are: pre-warnings, lateral shifts (discussed in the section for Local Street Projects), speed tables which can also be at intersections in the form of raised intersections (discussed in the section for Local Street Projects), and a technique called "reallocation of right of way".

Pre-warning techniques include the use of gateways, traffic circles at intersections, and the use of appropriate street edge treatments. Table 10, entitled Traffic Calming Tools of Complex Street Projects, Pre-Warning Devices, summarizes the measures and under what circumstances they should be utilized.

Table 10 - Traffic Calming Tools for Complex Street Projects
Pre-Warning Devices
Traffic Management Device Street Classification Design Speed
25 mph 30 mph
Gateways Collector
Minor Arterial
Yes Yes
Traffic circles Collector
Minor Arterial
Yes No
Street Edge Treatments Collector
Minor Arterial
Yes Yes
Roundabouts Minor Arterial No Yes

Gateways are devices that are used to signal to drivers that there is change in the environment ahead (both neighborhood and road). This can take the form of many things, including structures mounted in center islands, plantings, or a treatment that is distinctive but is safely placed. Again, pre-warnings have proven to be successful in providing warning for traffic calmed areas and have been shown to affect driver behavior as they proceed into the neighborhood.

Roundabouts (Figure 22) and traffic circles can serve as gateways or be placed intermittently at intersections as speed control measures. With landscaped center islands, they can introduce a change from a business area to neighborhood and serve as both physical and psychological dividers.

roundabout


Table 11, entitled Traffic Calming Tools of Complex Street Projects, Speed Tables and Lateral Shifts, summarizes these measures and when they should be utilized.

Table 11 - Traffic Calming Devices for Complex Street Projects
Speed Tables and Lateral Shifts
Traffic Management Device Street Classification Design Speed
25 mph 30 mph
Speed Table Collector
Minor Arterial (x)
Yes Yes (x)
Raised Intersection Collector
Minor Arterial (x)
Yes Yes (x)
Chicane Collector
Minor Arterial
Yes Yes
Chicane w/center Island Collector
Minor Arterial
Yes Yes
Lateral Shifts w/center Island Collector Yes Yes
Chokers Collector
Minor Arterial (x)
Yes (x) No
(x) - To be used in special cases with approval of the Traffic Division

Reallocation of Right-of-Way
addition or shift of on-street parking The technique of reallocation of right-of-way achieves speed control by redistributing space within the roadway right of way by giving motor vehicles lower priority and alternative modes a more equal or increased priority. Typically, this can include the reduction of pavement width to provide for easier pedestrian access at intersections or for a more pedestrian oriented street, the addition of bike lanes, the reduction of travel lane width, the addition of on-street parking, or the deflection of the roadway by a change in direction with possibly the addition of center islands. In the process of reallocation, one objective is to change the character of a road in ways that attempt to cause the motorist to drive more slowly and with greater alertness to potential conflicts. Sometimes, the effects or changes are as much psychological as physical because the motorists perceive that the road is no longer exclusively motor vehicle oriented. Table 12, entitled Traffic Calming Tools of Complex Street Projects, Reallocation of Right of Way, summarizes these measures and under what circumstances they should be utilized.

Street edge treatments can be particularly important in higher classification streets such as minor arterials, where other treatments might not be appropriate. These treatments are intended to provide a safe and pleasant environment for pedestrians and can provide a calmer environment for drivers. This includes the use of trees, which change the character of a street, reduce noise, provide shade, and reduce the optical width of a street.

In some cases, the techniques described above can be combined in traffic calming plans. Physically and as well as psychologically, this combination can result in improved driver behavior.

combined measures

Table 12 - Traffic Calming Devices for Complex Street Projects
Reallocation of Right of Way
Traffic Management Device Street Classification Design Speed
25 mph 30 mph
Bulbouts Collector
Minor Arterial
Yes Yes
Center Island Narrowings Collector
Minor Arterial
Yes Yes
Bike Lanes Collector
Minor Arterial
Yes Yes
Reduction Lane Width Collector
Minor Arterial
Yes Yes
Addition of Parking Collector
Minor Arterial
Yes (x) Yes
Street Edge Treatments Collector
Minor Arterial
Yes Yes
(x) - To be used in special cases with approval of the Traffic Division


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Last update 19 September 2000 - HvH