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Intersections

Article

Intersection physical and functional areas
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, crosswalks, bike lanes, and other roadside features. The image at right highlights the physical and functional areas of an intersection. Intersections are inherently points of conflict, where traffic converges and must be carefully designed to ensure safety and functionality for all modes.

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 roadway. Balancing the needs of all users at multimodal intersections, while maximizing substantive safety is a complex and important challenge.

Intersection diagram
Source: FHWA
  • Intersections are points of conflict where modes of travel converge, as illustrated in the image at right.
  • Intersections should include 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.

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 roadway. 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 that can be used to guide the design and construction of intersections and roadway improvements.

Intersections in Mixed-Use Districts and Transit-Oriented Developments (TOD)

The following is a list of characteristics influencing intersection design in mixed-use districts:

  • There is a significant transit presence
  • There is a significant pedestrian presence
  • Landscaped median Credit: CH2M HILL Multi-modal travel accommodations are needed

In addition to pedestrians, transit should be a prioritized travel mode in mixed-use areas. Transit presence, particularly the presence of buses, significantly influences the design of intersections.

As described in the capacity section, there are a variety of ways in which we can prioritize transit as a modal choice in mixed-use areas. Transit-only lanes and queue bypass lanes are two such elements, both of which require special treatment at intersections:

  • Transit-only lanes. These lanes provide dedicated space on the street for buses only (and sometimes other modes such as bicyclists or high-occupancy vehicles) and promote transit as a preferred mode by providing travel that is often more efficient than (private) vehicular travel. Transit lanes require special attention at intersections. Care must be taken to manage the conflict between transit-only lanes and right-turning vehicles, which essentially requires a lane transition. These transitions must be designed carefully to ensure that all users, including buses, are able to safely and efficiently execute their respective maneuvers. Clarity in pavement striping, signing, and taper design are important elements that should be used to accomplish effective designs for these transitions.
  • Transit bypass lanes or "queue jumpers" at intersections. Such treatments prioritize buses at intersections, allowing them to bypass congested queues forming in the vehicular travel lanes. Here again, care must be taken to manage the conflict between these lanes and right-turning vehicles at the intersection.

The relatively dense nature of mixed-use streets, combined with significant transit use, create high volumes of pedestrian activity. Providing the appropriate pedestrian facilities while maintaining an adequate level of efficiency for motor vehicle traffic is vitally important for mixed-use streets. 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.
  • 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. 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.
  • 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.

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 mixed use districts, 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.

The MUTCD signal warrants can be used to assess the appropriateness of a traffic signal along a mixed-use thoroughfare. 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.

Roundabouts are not appropriate in mixed-use areas where there are high volumes of pedestrian crossings. Roundabouts create a constant flow of vehicular traffic and impede pedestrian movement across the roadway.

Restrict turning movements during peak traffic. If traffic signals will be used at a particular intersection along the mixed-use street, several signal timing regulations can be implemented to improve vehicular and pedestrian operations. For example, limiting or prohibiting left turns, either throughout the day or at peak periods, can free up more "green time" for through traffic and improve vehicle operations. Prohibiting right turn on red helps maintain the primacy of service for pedestrians during walk intervals that correspond with red-light phases.

Use one-way streets strategically. One-way street systems can be used to limit the number of signal phases needed at intersections, allowing shorter cycle times and thus enabling better signal progression. However, some cities are converting existing one-way streets to two-way streets because their one-way systems were inappropriately planned and designed as mobility-oriented, motor-vehicle priority arterials. One-way streets in mixed-use areas must be designed to keep travel speeds relatively low and prioritize pedestrian travel.

Provide improved pedestrian facilities. Intersections are points of conflict and pedestrians are the most vulnerable users. Directional signage, pedestrian indicators, as shown at right, should be used to safely and efficiently guide pedestrians through signalized intersections in mixed-use corridors. Countdown clocks are a specific type of indicator using an active countdown display to communicate to pedestrians the time remaining to complete the crossing maneuver.

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 locator 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.

Keep curb radii small. Transit vehicles and users are typically more prevalent along mixed-use street corridors, and their presence should significantly influence the design of intersections, particularly curb return radii. Consistent with ITE's Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities, this guide recommends that curb return radii be as small as practicable in urban settings.

Consider channelized right turns when necessary and appropriate. In mixed-use street environments, the turning radii of transit buses should be used to select curb return radii only for intersections along designated bus routes. In these locations, additional design elements, such as channelized right turns, should be considered to enhance and protect pedestrian activity.

Chapter 10 of the ITE publication Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities provides additional information and guidance about channelized right turn lanes. While this treatment is not typically favored in areas with a significant pedestrian presence, well-designed channelized right turns can improve crossing conditions if and when large vehicles need to be accommodated. A well-designed channelized right turn should:

  • Reduce vehicular speed (5 to 10 mph is desirable) through the right turn movement.
  • Reduce the amount of information that pedestrians must process; this treatment allows pedestrians to examine the right turn lane first, then evaluate through traffic upon reaching the channelized island.
  • Offer a landing that protects pedestrians from through-moving vehicles; these landings are especially helpful for slower pedestrians that may not be able to make it across the entire street in one cycle.
  • Provide push buttons that are easy to access.
  • Improve signal timing for the intersection by reducing the pedestrian crossing distance.

When these channelized turning radii requirements are not feasible, an alternative is to keep curb return radii small but offset sidewalks, light poles, street furniture, and other streetside amenities, allowing the occasional large vehicle to ride over the curb to negotiate the turn without conflicts. Ensure curbs are designed for vehicles jumping them on occasion to minimize damage.