7.1 At-Grade Intersection Design and Roundabouts

7.1 At-Grade Intersection Design and Roundabouts

At-grade intersections are critical points in a highway network where traffic flows conflict. Design requires balancing capacity, safety, and physical space constraints. This section reviews key principles of channelization, design vehicle turning paths, island design, and roundabout geometry, referencing standard guidelines from the AASHTO Green Book (A Policy on Geometric Design of Highways and Streets) and the NCEES PE Civil Reference Handbook.

Principles of Channelization

Channelization is the separation or regulation of conflicting traffic movements into definite paths of travel by use of pavement markings, raised islands, or other suitable means. Well-designed channelization directs drivers into proper paths, minimizes confusion, and reduces conflict points.

Primary Objectives of Channelization:

1. Separation of Conflicts: Isolates specific turning movements (e.g., right-turn slip lanes) from through traffic.
2. Reduction of Conflict Areas: Minimizes the physical area where vehicles can collide, reducing exposure.
3. Control of Intersection Angles: Guides vehicles to intersect at or near 90 degrees (acceptable range is 75° to 105°). Right-angle intersections maximize visibility and minimize the crossing distance and time.
4. Discouragement of Undesirable Movements: Prevents drivers from making illegal or unsafe turns (e.g., left turns at restricted median openings).
5. Provision of Refuge Areas: Provides safe areas for pedestrians (refuge islands) and turning vehicles waiting for a gap.
6. Installation of Traffic Control Devices: Creates locations for signs, signals, and lighting.

Types of Islands

Islands are defined areas between travel lanes used to control vehicle movements. AASHTO classifies islands into three major categories based on function:

• Channelizing Islands: Guide traffic into specific paths, often used to separate right-turning traffic.
• Divisional Islands: Separate opposing traffic streams, often serving as medians to prevent head-on conflicts or restrict left turns.
• Refuge Islands: Positioned to aid pedestrians crossing wide streets, providing a safe midpoint.

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Turning Templates and Design Vehicles

A key constraint in intersection geometry is accommodating the physical dimensions and turning characteristics of the design vehicle. The design vehicle is the largest vehicle expected to use the intersection with frequency.

AASHTO Design Vehicles:

• Passenger Car (P): Minimum turning radius of 24 ft. Used for local residential streets.
• Single-Unit Truck (SU-30 / SU-40): Minimum turning radius of 42 to 45 ft. Used for collector streets and delivery routes.
• Intercity Bus (BUS-40 / BUS-45): Minimum turning radius of 45 to 50 ft.
• Interstate Semi-Trailer (WB-62 / WB-67): Minimum turning radius of 45 ft. These are the standard design vehicles for state highways and arterial intersections.

Swept Path and Offtracking

When a vehicle negotiates a turn, its rear wheels do not follow the same path as the front wheels. This phenomenon is known as offtracking (or low-speed offtracking).

• Swept Path Width: The physical width of the path envelope swept by the outermost front corner (e.g., front bumper) and the innermost rear wheel.
• Low-Speed Offtracking: Increases as the wheelbase increases and decreases as the turning radius increases.
• Design Application: Designers use turning templates (or CAD simulation software like AutoTURN) to ensure that the pavement width, lane widths, and curb radii are sufficient to prevent the design vehicle from mounting the curb or encroaching into opposing lanes.

Design vs. Check Vehicles

In intersection layout, engineers distinguish between the Design Vehicle and the Check Vehicle:

• Design Vehicle: Must be accommodated within the standard lane boundaries without encroaching into adjacent lanes or mounting curbs. The design vehicle is chosen based on daily traffic composition (e.g., standard buses on bus routes or WB-62 semi-trailers on industrial corridors).
• Check Vehicle: A larger, less frequent vehicle (e.g., an emergency fire engine or a WB-67 semi-trailer at a minor suburban intersection) that must be physically able to clear the intersection, but is permitted to encroach into adjacent or opposing lanes during its turn. This distinction prevents over-designing local intersections while ensuring emergency access.

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Island Design Guidelines

Raised curbed islands must be designed with sufficient size to be visible to drivers and command respect.

Size Requirements:

AASHTO establishes minimum size requirements for raised islands to ensure they are not easily overlooked by drivers:

• Urban Areas: Minimum area of 50 sq ft (preferably 75 sq ft).
• Rural Areas: Minimum area of 100 sq ft.
• Triangular Islands: Side lengths should be at least 12 ft (preferably 15 ft) after rounding corners.

Curb Offsets (Nose Offsets)

Curbed islands must be offset from the edge of the travel lane to prevent vehicles from striking the curb.

• Low-Speed Curbed Roads: The island nose should be offset at least 1 to 2 ft from the travel lane edge.
• High-Speed Roads (\ge 50 mph): The offset should be 2 to 3 ft.
• Approach Tapers: The nose of a divisional island should be gradually tapered to guide drivers away from the obstruction.

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Roundabout Geometry

A roundabout is a form of circular intersection where entering traffic yields to circulating traffic. Roundabout design is an iterative process prioritizing speed control, safety, and capacity.

Key Geometric Parameters:

Deflection and Speed Control

The most critical safety feature of a roundabout is deflection. Deflection is achieved by alignment offsets of the entering roads and the presence of the central island, which forces vehicles onto a curved path.

• Speed Limits: The design speed of a single-lane roundabout is typically 15 to 25 mph. For multi-lane roundabouts, the target entry speed is 25 to 30 mph.
• Fastest Path Analysis: Designers draw the fastest possible path a vehicle could take through the roundabout ($R_1$: entry path, $R_2$: circulating path, $R_3$: exit path, $R_4$: left-turn path, $R_5$: right-turn path). $R_1$ must be small enough to limit entry speeds to the target range.
• Splitter Islands: Raised or painted islands on each approach that separate entering and exiting traffic, provide pedestrian refuge, and assist in controlling entry deflection. They must be at least 100 ft long on high-speed approaches to give advance warning.

Fastest Path Analysis and Radius Relations

To control speeds, designers evaluate the "fastest path" a driver can take through the roundabout. This path represents the tightest radius a vehicle can travel along a smooth, continuous path assuming a driver ignores lane lines. Five critical radii are calculated:

1. Entry Path Radius ($R_1$): The minimum radius on the fastest path just before the yield line. This controls the entry speed.
2. Circulating Path Radius ($R_2$): The minimum radius on the fastest path around the central island.
3. Exit Path Radius ($R_3$): The minimum radius on the fastest path at the exit.
4. Left-Turn Path Radius ($R_4$): The radius of the path for a left-turning vehicle.
5. Right-Turn Path Radius ($R_5$): The radius of the path for a right-turning vehicle.

A major goal is to ensure that the difference in speed between consecutive curves is minimized to prevent crashes. The entering speed ($V_1$, associated with $R_1$) should be less than or equal to the circulating speed ($V_2$, associated with $R_2$). Typically, target fastest path speeds are:

• $V_1$ (Entry): 20 to 25 mph.
• $V_2$ (Circulating): 15 to 20 mph.
• $V_3$ (Exit): 25 to 30 mph. The exit radius $R_3$ is usually designed to be larger than $R_1$ and $R_2$ to allow vehicles to accelerate out of the roundabout, reducing congestion at the exit point.