9.3 Intersection and Corridor Planning

Intersection and Corridor Planning

Level of Service at Intersections

Intersections are the most complex points in a roadway network because they are locations where traffic streams cross, merge, or diverge. Consequently, they are the primary sources of vehicle delay and safety conflicts. The Highway Capacity Manual (HCM) evaluates intersection performance using control delay as the primary service measure. Control delay is defined as the total delay experienced by a driver due to deceleration, queue wait time, stopped delay, and acceleration back to free-flow speed.

Signalized Intersections

For signalized intersections, the Level of Service (LOS) is based on the average control delay per vehicle for the entire intersection, individual approaches, or specific lane groups. The delay thresholds are defined as follows:

• LOS A ($\le 10.0$ s): Very low delay. Most vehicles arrive during the green phase and proceed without stopping.
• LOS B ($10.0$ to $20.0$ s): Favorable progression. Short cycles and light queues.
• LOS C ($20.0$ to $35.0$ s): Moderate delay. Individual cycle failures (failing to clear the queue in one cycle) may begin to appear.
• LOS D ($35.0$ to $55.0$ s): Noticeable congestion. High volume-to-capacity ratios. Many vehicles stop.
• LOS E ($55.0$ to $80.0$ s): Limit of acceptable delay. Long cycle lengths, poor progression, frequent cycle failures.
• LOS F ($> 80.0$ s): Unacceptable delay. Demand exceeds capacity, queues grow continually, gridlock occurs.

Unsignalized Intersections (TWSC and AWSC)

For Two-Way Stop-Controlled (TWSC) intersections, the LOS is computed for each minor-street movement and major-street left turn (the major-street through movements have no delay). For All-Way Stop-Controlled (AWSC) intersections, the LOS is computed for the entire intersection. The thresholds are lower for unsignalized intersections than for signalized intersections:

• LOS A ($\le 10.0$ s)
• LOS B ($10.0$ to $15.0$ s)
• LOS C ($15.0$ to $25.0$ s)
• LOS D ($25.0$ to $35.0$ s)
• LOS E ($35.0$ to $50.0$ s)
• LOS F ($> 50.0$ s)

Why Unsignalized Thresholds are Lower

The HCM uses lower delay thresholds for unsignalized intersections for two reasons:

1. Driver expectation: Drivers expect to experience less delay at stop-controlled intersections than at traffic signals. A 40-second delay at a traffic signal feels acceptable, whereas a 40-second delay at a stop sign feels extremely frustrating.
2. Safety considerations: At stop-controlled intersections, drivers must identify gaps in oncoming traffic to make their turns. As delay increases, drivers become impatient and are more likely to accept unsafe, small gaps, leading to angle crashes. Traffic signals eliminate the need to judge gaps, so higher delays can be tolerated safely.

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Intersection Design and Channelization

The physical design of an intersection must facilitate safe and efficient traffic operations. Key design elements include channelization, corner radii, and design vehicle adjustments.

Channelization

Channelization is the separation of conflicting traffic movements into definite paths of travel by using traffic islands, raised medians, or pavement markings. The goals of channelization include:

• Directing traffic into proper turning paths.
• Preventing erratic or illegal maneuvers.
• Separating conflicting movements (e.g., separating left-turn lanes from through lanes).
• Controlling the angle of conflict (crossing movements should intersect as close to 90 degrees as possible to maximize visibility and reduce the conflict area).
• Providing refuge areas for pedestrians and left-turning vehicles.

Traffic Islands

Islands are raised or painted areas within the intersection. They are classified into three types:

1. Directional/Channelizing Islands: Guide turning vehicles into the correct path (most commonly seen as triangular islands that form a right-turn slip lane).
2. Divisional Islands: Separate opposing traffic streams on approach lanes (e.g., medians that prevent head-on crashes and guide movements).
3. Refuge Islands: Installed in the middle of wide cross-sections to give pedestrians a safe place to wait while crossing.

Turning Paths and Design Vehicles

Roadway geometry must accommodate the physical dimensions and operating characteristics of the vehicles using the facility, known as design vehicles. Design vehicles range from passenger cars (P) to single-unit trucks (SU-30) and large semi-trailers (WB-40, WB-50, WB-62, WB-67).

• Offtracking: When an articulated vehicle (like a tractor-trailer) negotiates a curve, its rear wheels do not follow the same path as its front wheels. Instead, the rear wheels pull inward toward the center of the turn. This phenomenon is called offtracking.
• Swept path width: The total width of the path swept by the front-most outer point and rear-most inner point of the turning vehicle.
• Curb Radii: Curb corner radii must be designed large enough to prevent the trailer wheels of the design vehicle from tracking over the curb, damaging traffic signs, or encroaching into adjacent lanes. However, too large a radius increases pedestrian crossing distance and vehicle speed. Engineers must balance these needs, often designing for a larger vehicle (e.g., WB-67) to occasionally encroach into adjacent lanes, while designing for smaller vehicles (e.g., SU-30) to stay within their lane.

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Sight Distance at Intersections (Sight Triangles)

To ensure safe operations, drivers stopped at or approaching an intersection must have an unobstructed view of the intersection and the crossroad. This clear area is called a sight triangle.

Approach Sight Triangles

Allow drivers approaching an uncontrolled or yield-controlled intersection to see potential conflicts and adjust their speed before reaching the intersection.

Departure Sight Triangles

Allow a driver stopped on the minor road to see approaching traffic on the major road in time to safely execute a maneuver (left turn, right turn, or crossing).

• The required length of the leg along the major road ($d$) is determined by the design speed of the major road ($V$) and the time gap ($t_g$) required for the design vehicle to accelerate and clear the conflict zone: $d = 1.47 \cdot V \cdot t_g$ Where:
  • $d$ = sight distance along the major road (ft).
  • $V$ = design speed of the major road (mph).
  • $t_g$ = time gap (seconds).
• AASHTO recommended time gaps for passenger cars:
  • Left turn from stop: 7.5 seconds (requires a larger gap because the vehicle must cross opposing traffic and accelerate).
  • Right turn from stop: 6.5 seconds.
  • Crossing maneuver: 6.5 seconds.
• If the major road is multilane, the time gap must be adjusted upward (typically by adding 0.5 seconds for each additional lane crossed for left turns and crossing maneuvers). For trucks (SU or WB vehicles), the time gaps are significantly larger (typically adding 2.0 to 4.0 seconds) to account for their slower acceleration.

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Roadway Functional Classification

Roadways are classified into a hierarchical system based on their role in the network. This system balances two competing objectives: mobility (high-speed, uninterrupted travel over long distances) and land access (direct access to residential driveways and commercial properties).

The Classification Hierarchy

1. Arterials: Prioritize mobility above all else. They are high-speed, high-capacity facilities designed for long-distance travel. Access is highly controlled (few driveways, grade-separated interchanges, or widely-spaced traffic signals). Divided into Principal Arterials (e.g., interstates, major freeways) and Minor Arterials.
2. Collectors: Collect traffic from local streets and feed them into the arterial network. They provide a balanced mix of mobility and land access. They typically have lower speeds and more frequent access points than arterials. Divided into Major Collectors and Minor Collectors.
3. Local Roads: Prioritize direct access to properties. They are low-speed, low-volume facilities. Through traffic is actively discouraged by geometric design (e.g., cul-de-sacs, winding layouts).

The Access-Mobility Trade-Off

A classic civil engineering concept is the access-mobility trade-off curve. As mobility increases, access control must increase, meaning direct land access decreases.

• An Interstate Freeway represents maximum mobility and zero direct land access.
• A residential cul-de-sac represents maximum land access and minimal mobility.
• Uncontrolled driveway access on arterials increases conflicts and crash rates. Therefore, corridor planning often involves access management (combining driveways, implementing raised medians to prevent left turns, or adding frontage roads) to preserve the arterial's mobility and safety.