3_3_reducing_speed

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Updated as of december 2010

Design Features that Reduce Operating Speeds

roadway designers should select design elements that reduce operating speeds to the desired values. as discussed earlier in this chapter, narrower lane widths have a traffic calming effect. the following speed-reduction strategies will be considered for roadway design on boston streets, and are therefore discussed in more detail on the pages that follow:

> midblock neckdowns and chicanes > center medians/islands > speed tables > paving treatments

the focus of this chapter is street design “between the curbs,” thus there are a variety of other chapters that should be referenced for other traffic calming strategies:

> Intersection treatments such as signal timing progression, raised intersections, and curb extensions can be used to reduce traffic speeds—these are discussed further in chapter 4: Intersections.

> street trees have a calming effect on traffic speeds – they are discussed in chapter 2: sidewalks.

> enforcement and regulatory measures can be used to reduce speeding—they are discussed in chapter 5: street management.

the design treatments on the following pages should be applied based on engineering judgment and in consultation with btd. the guidance in this document is the standard for the city of boston. It should be supplemented with design guidance provided in the manual on Uniform traffic control devices, the massdot project development, and design Guide and aasHto Greenbook.

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http://www.bostoncompletestreets.org/pdf/3/chap3_3_reduce_operating_speed.pdf



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Updated as of december 2010 71boston complete streets GUIdelInesboston transportatIon department

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Midblock Neckdowns

Overview

roadway geometry can be altered at midblock locations to reduce motor vehicle speeds by diverting the driver’s path of travel. neckdowns are curb extensions on opposite sides of the road which create a “pinch-point.” they are par-ticularly useful on streets with longer block lengths where motorists tend to pick up speed. they can be combined with midblock pedestrian crossings to further enhance pe-destrian safety by reducing crossing distance and increas-ing visibility.

Use

> neckdowns can be used on two-way streets with one lane in each direction, and one-way roads with no more than two lanes. they are sometimes combined with intermittent medians to reduce speeds along the length of a roadway.

> the amount of horizontal deflection should be based on the proposed design speed of the roadway.

> Vegetation used in the neckdown should generally be low-growing and low-maintenance. In locations with midblock pedestrian crossings, sight distances should be maintained.

the size, dimensions, and geometry of the neckdown will be based on the design speed of the road.

Considerations

> neckdowns can serve as alternatives in neighborhoods where noise associated with speed tables is a concern.

> care should be taken to avoid suddenly squeezing bicy-clists into the traffic flow on streets with higher volumes of traffic, particularly in locations with steep uphill grades where bicyclists may be travelling considerably slower than motor vehicle traffic.

> on low-volume residential streets, neckdowns can reduce the street to one lane, requiring on-coming drivers to alternate passage through the neckdown.

AppliCAble tO theSe StReet typeSneighborhood residentialneighborhood main streetdowntown commercialdowntown mixed-use

neighborhood connectorboulevard


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Midblock Chicanes

Overview

chicanes are curb extensions that alternate from one side of the street to the other, creating an “s” curve that drivers must weave through. chicanes can also be created by al-ternating parking from one side of the roadway to the other. chicanes provide opportunities to increase sidewalk space and introduce green street elements in the right-of-way.

Use

> chicanes and neckdowns can be used on two-way streets with one lane in each direction, and one-way roads with no more than two lanes.

> the amount of horizontal deflection should be based on the proposed design speed of the roadway.

> Vegetation used in chicanes should generally be low-growing and low-maintenance. In locations with mid-block pedestrian crossings, sight distances should be maintained.

sHared streetsneIGHborHood connectorboUleVardparkway

neIGHborHood resIdentIalneIGHborHood maIn streetdowntown commercIaldowntown mIxed-UseIndUstrIal

the size, dimensions, and geometry of the chicane will be based on the design speed of the road.

Considerations

> chicanes can serve as alternative in neighborhoods where noise associated with speed tables is a concern.

> on shared streets, chicanes can be created with street furniture and green element including trees.

> care should be taken to avoid suddenly squeezing bicy-clists into the traffic flow on streets with higher volumes of traffic, particularly in locations with steep uphill grades where bicyclists may be travelling considerably slower than motor vehicle traffic.


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sHared streetsneIGHborHood connectorboUleVardparkway

Center islands neIGHborHood resIdentIalneIGHborHood maIn streetdowntown commercIaldowntown mIxed-UseIndUstrIal

Overview

a center island can be used at a midblock location to narrow the roadway, reduce motor vehicle speeds and improve pedestrian crossings. Studies show that intermittent (mid-block) islands can result in up to a 7% reduction in motor vehicle speeds. center islands also provide opportunities to introduce green elements in the right-of-way, and can be used to absorb stormwater and reduce the heat island effect.

Use

> Islands with pedestrian refuge improve pedestrian safety and access by reducing crossing distances and enabling pedestrians to cross roadways in two stages. medians can also be designed to accommodate sidewalks and shared-use paths.

> center islands can reduce the risk of head-on collisions and limit left turn opportunities to desirable locations (e.g., signalized intersections).

> Islands designs should be carefully designed to ensure proper drainage and maximize the potential for on-site stormwater retention and infiltration. median designs should be appropriate to the roadway context. for ex-ample, raised medians are generally more appropriate for urban settings while depressed medians are generally more appropriate for suburban settings.

Considerations

> sidewalks should not be reduced in width and bike lanes should not be eliminated in order to provide space or ad-ditional width for islands.

> center medians can be combined with midblock pedes-trian crossings to reduce crossing distances (see page x).

> permeable surfaces, street trees, and low-growing (less than 3’ in mature height including the height of the curb and earthwork), drought-resistant plant materials should be used wherever safe and feasible.

> plants should be located as far from the curb as possible to prevent injury from salt, sand, and snow.

> landscaped medians require regular maintenance. a main-tenance agreement establishing responsibilities maintain plantings is essential.

> medians should be at least 6 feet wide when used for low plantings, 10 feet wide for columnar trees, and 18 feet wide for larger shade trees.


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Speed tables sHared streetsneIGHborHood connectorboUleVardparkway


Overview

speed tables are raised pavement areas that are placed at midblock locations to reduce vehicle speeds. speed tables are elongated and have been shown to effectively reduce 85th percentile speeds by 13-15 mph. well-designed speed tables enable vehicles to proceed comfortably over the device at the intended speed, but cause discomfort when traversed at inappropriately high speeds.

Use

> speed tables are usually 3 inches higher than the roadway surface and 3 inches below the top of the curb and generally 10 to 14 feet in length. they normally extend the full width of the roadway, although sometimes they are tapered at the edges to accommodate drainage patterns.

> speed tables should be designed with a sinusoidal profile, which provides a smoother transition for bicyclists, and a smooth leading edge.

> speed tables should be clearly marked with reflective pavement markings (per the mUtcd) so that motorists and bicyclists are aware of their presence and can adjust their speed accordingly.

speed tables typically are 3-4 inches higher than the roadway, and 10-14 feet long.

Considerations

> speed tables should not be confused with speed bumps. Speed bumps are used in parking lots and are not recommended for public streets.

> longer speed tables (up to 22 feet in length) have a design speed of 25 to 30 mph and are easier for large vehicles to negotiate.

> avoid placing speed tables at the bottom of steep inclines where bicyclists travel at higher speeds and may be surprised by their presence.

> speed tables should be supplemented with other traf-fic calming measures to effectively reduce travel speeds throughout a corridor, or neighborhood. when used alone, speed tables may otherwise result in speed “spiking”; motorists may travel at higher speeds between tables.

> speed tables should be located in consultation with the boston fire department and ems.


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stamped asphalt

textured asphalt

stamped concrete


paving treatments sHared streetsneIGHborHood connectorboUleVardparkway


Overview

special paving treatments can be used on roadway surfaces to reduce speeds. this strategy is more common on streets with high volumes of pedestrians and lower volumes of motor vehicle traffic, such as shopping districts and main streets. boston’s historic cobblestone streets are an example of the effects of textured pavements on vehicle speeds. modern textured pavements are smoother than cobblestones to ac-commodate bicyclists. regardless of the material used on the roadway, an accessible, smooth travel path must be provided at crosswalks in order to accommodate people with disabili-ties. additional discussion of paving treatments is included in roadway materials in this chapter.

Use

> paving treatments can include stamped concrete and asphalt and textured asphalt.

> pavers may be used in historic districts. > care must be taken to ensure textured pavements are structurally sound and able to support the type and volume of vehicles that are likely to use the street.

> care should also be taken to ensure that materials do not settle to different heights.

> pedestrian crossings must meet ada by providing a smooth, slip-resistant surface, and should include the necessary reflective markings as required in the mUtcd. see Intersections for guidelines on the use of different pav-ing materials at intersections.

Considerations

> textured pavements are an expensive treatment and include long-term maintenance responsibilities.

> the use of pervious and low glare pavements is covered in chapter 6.

> slippery surfaces such as smooth granite, tile, or brick should not be used as they create slippery conditions for bicyclists and pedestrians in wet weather.

> noise can be a concern with textured pavements.

> the use of concrete bus pads should be considered on high frequency bus routes.

> Use of paving treatments in parking lanes can visually reduce the width of the roadway.


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