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.
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.
The following is a list of characteristics influencing intersection design in civic and educational corridors:
Civic and educational corridors have 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 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:
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.
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 thoroughfare, as shown at right. 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 these environments.
Roundabouts are not appropriate in 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, 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 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 and pedestrian indicators, as shown at right, should be used to safely and efficiently guide pedestrians through signalized intersections. 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:
Keep curb radii small. Transit vehicles and users are typically more prevalent along these 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. When transit buses are common, 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:
When choosing channelized right turns at an intersection, consider implementing raised crosswalks through the right-turn lane. Doing so has a traffic-calming effect on right-turning vehicles and makes the crossing easier for wheelchair pedestrians (curb ramps are unnecessary).
When 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.
In addition to pedestrians, transit should be a prioritized travel mode in these 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 these areas. Transit-only lanes and queue bypass lanes are two such elements, both of which require special treatment at intersections:
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