Intersections

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Source: FHWA An intersection is defined as the area where two or more roadways join or cross, but also includes elements of the functional area, such as intersection approaches, medians, sidewalks, bike lanes, and other roadside features.

The image at right highlights the physical and functional areas of an intersection. Intersections on great streets must serve all modes of travel.

Automobiles, transit vehicles, pedestrians, and bicyclists should all be given adequate time, space, and directional cues to safely proceed through intersections and continue traveling along the arterial. Balancing the needs of all users at multimodal intersections, while maximizing substantive safety is a complex and important challenge.

Source: FHWA

Intersections are points of conflict where modes of travel converge, as illustrated in the image at right.
Intersections should be carefully designed to include and prioritize the most appropriate place-specific design elements.
Intersecting roadways should cross at an angle of at least 75 degrees, ideally 90 degrees. When the angle of intersection is less than 60 degrees special design treatments may be needed to ensure a reasonable level of safety.
At intersections, medians can be used to provide separation between opposing traffic, channelization for turn lanes, and refuge for pedestrians.
Medians with landscaping and tree plantings can also be used to improve intersection (or roadway) aesthetics, although care should be taken not to affect driver or pedestrian visibility and sight distance.

Credit: FHWA

Movement through intersections is controlled using yield signs, stop signs, roundabouts, and traffic signals. The appropriate type of control for a given intersection depends on the place type and the amount of pedestrian and vehicular traffic.

The MUTCD provides guidance for selecting the appropriate type of control for various intersection conditions (see the following links for general information and specifics about signal warrants).

Different traffic control devices impose varying degrees of delay on pedestrians and vehicles passing through the intersection. The overall efficiency and capacity of a roadway is limited by the delay experienced at its intersections.

Some agencies and municipalities continue widening intersections by adding exclusive, dual, or even triple turn lanes in an effort to minimize delay along the arterial. While these improvements do increase an intersection's vehicular capacity, they also render the intersection more difficult for other modes of travel (especially pedestrians) to navigate. Because turn lane additions are typically retrofit projects they can significantly impact surrounding residences, businesses, and land parcels.

Designing intersections for great streets requires balancing competing needs, interests, and values, and responding to the unique circumstances of each street. Planners, designers, policy makers, and local stakeholders should collaborate to develop a community vision which can be used to guide the design and construction of intersections and roadway improvements.

Characteristics that influence intersection design for residential thoroughfares:

Significant pedestrian presence
Lower speeds desired
Single land use (residential), smaller parcel sizes
Quality of life, pedestrian convenience, and safety are all important drivers

Residential thoroughfares have high volumes of pedestrian activity. Providing the appropriate pedestrian facilities is vitally important for these thoroughfares. Roadway and intersection design should reflect this need for a safe, attractive, and comfortable pedestrian environment.

AASHTO's Guide for the Planning, Design, and Operation of Pedestrian Facilities offers the following as characteristics of good intersection design:

Clarity - Motorized traffic should be alerted to the presence of pedestrians; pedestrians should be able to easily identify crossing locations; both goals can be achieved using appropriate sign placement and design. Noticeable textures and colors can also be used to emphasize crosswalks for pedestrians and drivers approaching the intersection.
Predictability - Place pedestrian crossings in expected or predictable locations. In unexpected locations, use clear and visible signing, flashing lights, or beacons to alert drivers and pedestrians of the crossing.
Visibility - Providing adequate sight distance and appropriate lighting can improve visibility for both pedestrians and motorists. The sight distance required at an intersection is based on the design speed of the facility and constrained by various objects along the roadway (e.g. bus stop shelters, street furniture, utilities, building corners) as well as the street's vertical curvature. Although most mixed-use streets have relatively low design speeds, roadway planners and engineers should consider selecting sight distances for higher speeds to further increase visibility.
Short Wait - Minimize the time pedestrians spend waiting to cross an intersection. Consider special peak hour signal timing to meet the high pedestrian demand during these time periods.
Sufficient Crossing Time - Signals should be programmed to ensure that all users, including the elderly and individuals with disabilities, have adequate time to safely cross the intersection. Walking speed assumptions in elementary school areas should be more conservative to account for younger children crossing the street. Newer pedestrian signals, such as the one shown in the image at right, provide countdown clocks which clearly communicate to pedestrians the time remaining to complete the crossing. Here again, the high pedestrian demand during peak hours warrants consideration of special peak hour signal timing to truly prioritize pedestrians.
Limited Exposure - Reducing crossing distance, providing refuge islands, and reducing conflict points can minimize a pedestrian's exposure to traffic while crossing an intersection.

Intersections should be as compact as possible in order to minimize crossing distances for pedestrians. For larger intersections, mid-street refuge islands allow pedestrians to cross one lane or direction of traffic at a time. Right-turn-on-red restrictions can also be used to reduce pedestrian exposure in the crosswalk.

On streets with curbside parking, curb extensions can reduce the required crossing distance and time. Curb extensions, as shown in the image at right, can also make pedestrians more visible to drivers.
Clear Crossing - The crossing path, including sidewalk ramps adjacent to the street, should be clear of all barriers, including utility poles, fire hydrants, and signalization equipment. The crossing path must also be ADA compliant. Compliance is generally an opportunity to enhance intersections with amenities that are both inclusive and attractive, as shown at right.
Enforcement - Presence of security and law enforcement during peak travel periods may be necessary to enforce the low-speed, pedestrian-prioritized environment necessary for these thoroughfares. This is especially true when trying to transform areas to reflect "new" values.

Consider pedestrian presence when selecting the type of control at intersections. Traffic signals, signs, and markings are used to guide and regulate the multi-modal interaction and movements at intersections. Chapter 2 of the MUTCD discusses the merits of several control measures and describes the warrants for each. For example, stop signs are typically used on minor roads intersecting the arterial street in residential areas, as shown at right. Although stop signs can also be used on major arterials, intersections must be carefully designed to ensure that pedestrians waiting to cross are clearly visible and motorists yield the right-of-way when pedestrians are present.

Generally speaking, traffic signals are less common along residential thoroughfares than the others considered as part of this guide. Even so, the MUTCD signal warrants can be used to assess the appropriateness of a traffic signal along residential thoroughfares where they intersect with other higher-volume streets. Effective warrant evaluation necessitates the use of current, comprehensive data for vehicular and pedestrian traffic and direct field observation by the individuals ultimately making the traffic control recommendations.

Meeting some or all of the eight warrants outlined in the manual does not mandate the use of a traffic signal, but this information should be used by local leaders, planners, and designers in decision-making. Warrant 4, in particular, focuses on pedestrian demand and should be given special attention in mixed use environments.

If traffic signals or mid-block pedestrian crossings are present, they should include Accessible Pedestrian Signals (APSs). APSs provide various types of information to pedestrians with vision impairments. APSs can help create great streets that are accessible for all users. Chapter 4E of the MUTCD provides additional information on APSs and their application.

AASHTO's Guide for the Planning, Design, and Operation of Pedestrian Facilities, describes several types of APSs, including:

Audible at Pedestrian Signal Head: a speaker on top of the pedestrian signal head emits a bell, buzzer, cheep, spoken message, or some other audible tone during the walk interval, alerting pedestrians of the appropriate time to cross.
Audible at Push Button: a locater noise is constantly emitted from the push button to identify its location. When the button is pushed, it triggers the emission of a voice message or other noise signal when the walk interval begins.
Vibrotactile: the push button or arrow vibrates during the walk interval, allowing those who cannot see to feel the vibration and know that the walk interval is active.
Transmitted Message: pedestrians wearing a special receiver can hear intersection-specific information, such as the announcement of walk intervals, which is transmitted from an infrared or LED device on the signal head.

Due to the primacy of pedestrians in residential areas, lower vehicular speeds are desirable along the thoroughfare. Lower speeds create less noise and improve the quality of life for residents living along the street. They also make it more convenient and safe for pedestrians along the street.

There are a variety of other intersection design treatments that can help contribute to lower speed environments:

Credit: Charlier Associates

Neighborhood traffic circles (not to be confused with modern roundabouts) are physical circles built in the middle of intersections to slow vehicular speeds down through the intersection.

They prevent through-traffic on the arterial street from driving straight through the intersection by forcing drivers to alter their path of travel around the circle.

If considering neighborhood traffic circles as an intersection design treatment, the following points should be kept in mind:

Landscaping, public art, water fountains, and other treatments can be used in the center of the traffic circle to add aesthetic value to the street. While they certainly can be aesthetically very pleasing, the height of such treatments can impede visibility for pedestrians at the intersection as they try to cross the various legs of the street.
Nighttime lighting in residential neighborhoods is often intentionally darker than other arterial streets to accommodate local residents. This can make it difficult to clearly see neighborhood traffic circles as drivers approach the intersection. Raised cobble stone strips or other such raised pavement treatments in advance of the intersection can be an effective way to alert drivers to the approaching intersection treatment. Advanced signing located in areas of clear visibility are also important in this regard.
Intersections with high left turn volumes should not consider neighborhood traffic circles due to the awkward geometry constraining the left turn movements. Left turning vehicles tend to cut through the left side instead of driving around the circle.

Modern roundabouts can also be an effective traffic calming method for residential streets. Figure 1 below is a generic depiction of typical modern roundabout features:

Modern roundabouts are very different from neighborhood traffic circles. While they both have "circles in the middle of the intersection, they differ substantially on the size of that central island and in the curvature of the intersection approaches and intersection corners. These features are designed to promote the desired speed through the roundabout, in this case lower speeds. Table 1 provides typical values for a variety of roundabout types. Type selection should be made based on the characteristics of the respective residential street. For the residential place types considered in this guide, the rural categories are not applicable.

More detail on roundabouts is provided in FHWA's Roundabouts: An Informational Guide.

Curb extensions are another intersection treatment that can have a traffic calming effect in residential neighborhoods. Curb extensions narrow the width of the road at the intersection, causing drivers to slow down as they approach. They also decrease the length of crossing distance required for pedestrians.

The design of corner curvature, or curb return radii, at intersections has a direct effect on travel speed. Smaller radii create a sharper turn and force drivers to slow down as they navigate the turn. Larger radii have the opposite effect, allowing drivers to navigate the turn at higher speeds. Smaller radii, therefore, are more desirable for residential streets in an effort to keep travel speeds low.

Credit: Charlier Associates

Bicycle travel should be encouraged and prioritized in residential place types. This mode is an efficient, environmentally responsible choice that provides residents a very economical way to make short trips, presuming that good bicycle accommodations are available.

Treatment of bicycle lanes at intersections can be a delicate balancing act due to the variety of conflicts that bicyclists will encounter, as shown at right. In residential areas, bicycle travel should be prioritized at these locations to ensure user safety and to maximize the attractiveness of the bicycle mode choice.

Various intersection treatments are available to accommodate vehicular turning movements while maintaining the integrity of the bicycle lane facility. It is generally appropriate to dash or drop the bike lane striping where merge movements will occur across the bicycle lane. See Chapter 9C of the MUTCD and the AASHTO Guide for the Development of Bicycle Facilities for specific guidelines.
Curb extensions used for traffic calming and pedestrian benefits should not extend into the bicycle lane.

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Last Updated on Tuesday, 10 March 2015