13.2 Design Considerations

13.2.1 Intersection Characteristics

An intersection includes the areas needed for all modes of travel that use the intersection. These modes of travel include:
  • Bicycle/Micromobility;
  • Wheelebase (WB);
  • Pedestrian;
  • Truck (WB Trucks, Recreational Vehicles, etc.); and
  • Transit (School Bus, City Bus, Trains, etc.).
Therefore, the intersection design not only addresses the roadway pavement, but the adjacent sidewalks, pedestrian curb ramps, bicycle facilities, speed-change/auxiliary lanes, medians, and islands. Intersections are a key feature of roadway design in four respects:
  • Focus of Land Use Activity –
    The land near intersections often contains a concentration of travel destinations that are accessed by multiple modes of transportation. This is especially true for intersections in the Urban and Urban Core Contexts.
  • Conflict Points –
     Pedestrians, bicyclists, and motor vehicles often cross paths at intersections where through and turning movements conflict. These crossings are referred to as “conflict points” and can be further categorized by movement type and severity.
  • Traffic Control –
     Movement of users may be assigned through use of traffic control devices such as yield signs, stop signs, and traffic signals.
  • Capacity –
     Traffic control at intersections will often limit the number of users that can be accommodated within a given time period on intersecting roadways.
Figure 13-1
shows the number and type of motor-vehicle conflict points at typical four-leg, three-leg, and roundabout intersections. Conflict points should also be considered when locating driveways along a roadway. Providing separation between driveways reduces the potential for collisions by reducing the number of conflict points and increasing the distance between conflict points. See
Chapter 16
and refer to TxDOT’s
AMM
for additional information on driveway spacing requirements and driveway design.
Conflict Points at Various Intersection Types ( click in image to see full-size image)
Figure 13-1: Conflict Points at Various Intersection Types
Source: AASHTO’s A Policy on Geometric Design of Highways and Streets

13.2.2 Intersection Functional Area

An intersection is defined as both its functional and physical areas, as illustrated in
Figure 13-2
. The functional area extends both upstream and downstream from the physical intersection area and include any auxiliary lanes and their associated channelization. The functional area on the approach to an intersection or driveway is comprised of the following three fundamental components:
  • perception-reaction distance,
  • deceleration distance, and
  • storage length as illustrated in
    Figure 13-3
    .
The perception reaction distance represents the distance traveled while a driver recognizes the upcoming turn lane and prepares for a maneuver. The distance covered during the perception-reaction period is contingent on factors such as vehicle speed, driver attributes, and driver familiarity with the location. Refer to
AASHTO’s A Policy on Geometric Design of Highways and Streets
for additional information on perception reaction distance and the time values used for each context classification.
In cases where there are dedicated left or right-turn lanes, the deceleration distance encompasses the space required for both deceleration and lane change. In the absence of turn lanes, it entails coming to a smooth stop. The storage length should be sufficient to accommodate the longest queue expected for the majority of the time. Ideally,
driveways should not be situated within the functional area of an intersection,
 as demonstrated in
Figure 13-2
, or within the influence region of an adjacent driveway.
Physical and Functional Area of an Intersection ( click in image to see full-size image)
Figure 13-2: Physical and Functional Area of an Intersection
Source: AASHTO’s A Policy on Geometric Design of Highways and Streets
Elements of the Functional Area of an Intersection ( click in image to see full-size image)
Figure 13-3: Elements of the Functional Area of an Intersection
Source: AASHTO’s A Policy on Geometric Design of Highways and Streets

13.2.3 Design Objectives

The goal of any intersection design, regardless of type or location, should be to implement the following principles:
  • Reduce vehicle speeds through the intersection as appropriate;
  • Provide the appropriate number of lanes and lane assignment to achieve adequate capacity, lane volume and lane continuity;
  • Provide channelization and appropriate signage that operates smoothly, is intuitive to drivers, and results in vehicles naturally using the intended lanes;
  • Provide suitable intersection radii so all turning movements are acceptable for the design vehicle;
  • Meet the needs of pedestrians and bicyclists; and
  • Provide appropriate sight distance and visibility.
Each element described above influences the operational efficiency and potential for crashes at intersections. When developing an intersection design, the appropriate balance of operational performance for various modes, safety, and cost considerations should be sought throughout the design process. Favoring one component of the design may negatively affect another.
The design of each intersection should achieve an appropriate balance among the competing needs of pedestrians, bicyclists, motor vehicles, and transit with respect to safety, operation efficiency, convenience, ease, and comfort.
Four basic elements should be considered in intersection design:
  1. Human Factors
    • Driving habits;
    • Ability for users to make decisions;
    • User expectancy;
    • Decision and reaction time; and
    • Pedestrian and bicyclist behavior.
  2. Traffic Considerations
    • Functional Classification of each intersecting roadway;
    • Existing and expected future crash frequency and severity;
    • Design and actual capacities for all modes of traffic;
    • Design-hour turning movements;
    • Size and operating characteristics of vehicles and modes;
    • Potential conflicts between transportation modes;
    • Vehicle Speeds;
    • Transit usage and stop locations; and
    • Railroad crossing accommodation, if applicable.
  3. Physical Elements
    • Character and use of abutting property;
    • Available right of way (ROW);
    • Pedestrian and bicyclist facilities;
    • Transit facilities, if applicable;
    • Vertical and horizontal alignments at the intersection (see
      Section 4.8.5
      );
    • Sight distance;
    • Angle of the intersection;
    • Speed change lanes;
    • Traffic control devices;
    • Lighting;
    • Roadside design features;
    • Crosswalks;
    • Adjacent driveways and driveway spacing;
    • Drainage considerations; and
    • Provision for utilities.
  4. Economic Factors
    • Cost of improvements and expected benefits; and
    • Cost and effectiveness of controlling access points to abutting residential or commercial properties.
Refer to
AASHTO’s A Policy on Geometric Design of Highways and Streets
for additional design objectives and considerations.

13.2.4 Design Considerations for Intersection User Groups

The design of intersections should use performance measures and engineering judgement to balance the needs of all roadway users and transportation modes. The size and design of physical elements such as roadway width, lane width, and corner radii are selected according to the volume and priority of each of the intersection user groups.
For an intersection in the urban and urban core contexts, design priority may be given to pedestrians, bicyclists, passenger vehicles, and buses. Basic accommodation may be given to trucks, except for intersections located on truck routes or Freight Network. Intersections in the suburban or rural contexts near industrial and commercial areas may be designed for automobiles and trucks with basic accommodations for pedestrians, cyclists, and transit. Design considerations for users include:
Motor-Vehicles (not including trucks) –
 Key elements affecting intersection performance for motor vehicles are:
  • The type of traffic control;
  • Vehicular Capacity of the intersection (i.e., number of lanes);
  • Ability and Capacity to make turning movements;
  • Visibility of approaching and crossing pedestrians and bicyclists; and
  • Visibility of approaching and crossing motor vehicles.
Bicyclists –
 Key elements affecting intersection performance for bicyclists are:
  • The degree to which the roadway pavement is shared or used exclusively by bicyclists;
  • The relationship between turning and through movements for motor vehicles and bicyclists;
  • Traffic control for bicyclists;
  • The differential in speed between motor vehicles and bicycles; and
  • Conflicts with pedestrian movements
Pedestrians –
 Key elements affecting intersection performance for pedestrians are:
  • The amount of ROW provided for pedestrians use (e.g., sidewalk width);
  • Crossing distance and resulting duration of exposure to motor-vehicle and bicycle traffic;
  • The volume of conflicting traffic;
  • Speed and visibility of approaching traffic;
  • Turning speeds;
  • Permissive right-turn on red;
  • Permissive left turn movements;
  • Accessibility to pedestrians with disabilities; and
  • Traffic Control for Pedestrians
Trucks –
 Trucks share many of the same elements as motor-vehicles described in the preceding sections. In addition:
  • Trucks are typically three or four times the length of other motor-vehicles;
  • May be much slower starting than most motor-vehicles; and
  • May need much larger turning radii than most motor-vehicles.
Therefore, the presence and frequency of trucks affects the capacity of the intersection, the width of the driving surface needed for turning movements, and the radius of turning movements.
Design of intersection elements for one group of users will often affect other users.
 For example, an intersection designed to accommodate trucks with no encroachment into adjacent lanes needs large corner radii, wide turning roadways, and results in greater distances for pedestrians to cross.
In addition to the users of the street and intersections, owners and users of adjacent land often have a direct interest in the intersection design. This interest can be particularly sensitive where the intersection is surrounded by retail, commercial, historic, or institutional land uses. The primary concerns include:
  • Maintenance of vehicular access to private property;
  • Turn restrictions;
  • Landscaping;
  • Driveway access;
  • ROW acquisition; and
  • Provision of convenient pedestrian and bicyclist access.

13.2.5 Intersection Capacity

The capacity of a roadway to serve motor vehicles is determined primarily by constraints that are present at intersections. Vehicles turning to and from the primary roadway at unsignalized intersections cause through vehicles to stop or slow, thereby influencing traffic flow and affecting the level of service. The available green time at signalized intersections for any given traffic movement is substantially less than would be available for free-flow operations. Therefore,
it is important to analyze and characterize the combined effects of geometry and traffic control proposed for the design of an intersection.
Refer to the
TSAP
and the
HCM
for additional information and operational analysis techniques for intersections.