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Intersections

Article

Physical and functional areas of an intersection 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.

Intersections and conflict
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.
Landscaped median
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 to widen intersections by adding exclusive, dual, or even triple turn lanes in an effort to minimize vehicular 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.

Intersections along Small Town Downtown Corridors

The following is a list of characteristics influencing intersection design for small town downtown streets: 

  • Congestion is more tolerable and expected;
  • There is a significant pedestrian presence

The primary challenge in designing intersections in small town downtown areas is meeting the needs of pedestrians while maintaining an adequate level of efficiency for motor vehicle traffic. Pedestrian traffic is a primary mode in small town downtown areas. 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 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 small town downtown 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.
  • Pedestrian signal Credit: CH2M HILL 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.

    Curb extension
    Credit: CH2M HILL

    On streets with curbside parking, curb extensions can reduce the required crossing distance. 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.

Compliant ramp Credit: CH2M HILL 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 small town downtown environments, as shown at right.

Pedestrians in crosswalk
Credit: CH2M HILL

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 downtown arterial street.

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 small town downtown environments.

Roundabouts are not appropriate in downtown 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 small town downtown 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.  

Pedestrian refuge
Credit: CH2M HILL

Provide improved pedestrian facilities. Intersections are points of conflict and pedestrians are the most vulnerable users. Directional signage and pedestrian indicators should be used to safely and efficiently guide pedestrians through signalized intersections in small town downtown 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.  Large vehicles such as trucks and buses are less common in small town downtowns.  When present, they are usually passing through town as opposed to turning onto one of the cross streets or driveways.  It is therefore unnecessary to have large curb return radii.  It is recommended to keep these radii as small as practicable.  Doing so forces turning vehicles to significantly reduce speeds when turning around a corner.  The reduced speed conditions are desirable in these place types due to high pedestrian presence.

Consider channelized right turns when necessary and appropriate. In small town downtown environments, there are occasionally those driveways or cross streets that do require large truck access. In these select 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.  Raised crosswalks are also recommended as a way to both slow turning vehicles and minimize the grade change experienced by wheelchair pedestrians.
  • 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.