Module 9: Intersection and Interchange Geometry on Mohammad Movahedi

Intersection Sight Distance

Mon, 04 May 2026 00:00:00 +0000

Intersection Sight Distance

Intersection Sight Distance (ISD) is the sight distance required at an intersection to allow drivers to safely perceive and react to potential conflicts, enabling them to cross or merge with opposing traffic streams. ISD is evaluated using the concept of sight triangles, which define unobstructed areas along the intersection’s corners.

Sight Triangles

Sight triangles represent the clear zones required for drivers to see approaching vehicles. There are two types:

At-Grade Intersection Layout

Mon, 04 May 2026 00:00:00 +0000

At-Grade Intersection Layout

At-grade intersection layout involves the geometric design of intersections where two or more roadways cross at the same elevation. The layout must balance the competing needs of through vehicles, turning vehicles, pedestrians, bicyclists, and transit vehicles.

Horizontal and Vertical Alignment

1. Intersection Angle

The angle of intersection is the angle between the centerlines of the intersecting roadways.

Ideal Angle: $90^\circ$ (perpendicular).
AASHTO Recommendation: Intersections should intersect at or near $90^\circ$ whenever possible. A skew angle of up to $60^\circ$ ($30^\circ$ deviation from perpendicular) is the absolute maximum recommended skew.
Consequences of High Skew (skew angle $< 60^\circ$):
- Blind Spots: Drivers must turn their heads at extreme angles to see oncoming traffic.
- Longer Crossing Distance: The width of the intersection increases, which increases pedestrian crossing times and the time vehicles are exposed to conflicts.
- Larger Sight Triangles: Due to the skew, sight triangles must cover a larger area.
- Swept Path: Turning vehicles require wider swept paths, increasing the likelihood of lane encroachment.

Perpendicular (90°)

Skewed (60°)

2. Vertical Profile

Roadway profiles near intersections should be as flat as possible to allow drivers to stop safely and accelerate from a stop.

Turn Lane Design

Mon, 04 May 2026 00:00:00 +0000

Turn Lane Design

Auxiliary turn lanes (left-turn and right-turn lanes) are added to intersections to remove decelerating or stopped turning vehicles from the through-traffic stream. This increases the capacity of the intersection, reduces delays, and lowers the potential for rear-end collisions.

Components of an Auxiliary Turn Lane

An auxiliary lane consists of three distinct horizontal segments:

Entering Taper ($L_{\text{taper}}$): The transition zone where vehicles lateral into the auxiliary lane. It can be a straight-line taper or a curved transition.
Deceleration Length ($L_{\text{decel}}$): The distance required for a vehicle to decelerate from the through-lane design speed to a stop (or to the speed of the turning maneuver) before reaching the queue of stored vehicles.
Storage Length ($L_{\text{storage}}$): The length reserved for vehicles waiting to complete the turn.

Design and Calculations

1. Entering Taper Length ($L_{\text{taper}}$)

For a straight-line transition, the taper length is:

Driveway and Access Management

Mon, 04 May 2026 00:00:00 +0000

Driveway and Access Management

Access management is the systematic control of the location, spacing, design, and operations of driveways, median openings, interchanges, and street connections to a roadway. The primary objective is to balance the competing needs of roadway mobility (moving through traffic safely and efficiently) and land access (providing entry to roadside properties).

Core Principles of Access Management

AASHTO and the Federal Highway Administration (FHWA) identify several foundational principles of access management:

Roundabout Geometric Design

Mon, 04 May 2026 00:00:00 +0000

Roundabout Geometric Design

A modern roundabout is an at-grade intersection where traffic circulates counterclockwise (in the United States) around a central island. Unlike older traffic circles or rotaries, modern roundabouts operate under yield control on entry, require deflection to slow entering vehicles, and utilize a compact design to limit speeds and improve safety.

Key Geometric Features

A modern roundabout contains several distinct geometric elements:

Inscribed Circle Diameter (ICD): The overall outer diameter of the roundabout, measuring from the outer edge of the circulatory roadway.
- Single-Lane Roundabouts: Typically $90$ to $150\text{ ft}$ ($27$ to $45\text{ m}$).
- Multi-Lane Roundabouts: Typically $150$ to $220\text{ ft}$ ($45$ to $67\text{ m}$).
Central Island: The non-traversable circular area in the center of the roundabout.
Truck Apron: A mountable, low-profile concrete ring surrounding the central island. It is designed to accommodate the rear wheel tracking of large trucks (e.g., WB-67) while keeping the entry and circulatory lanes narrow to restrict the speeds of passenger cars.
Splitter Island: A raised or painted triangular island on each approach. It serves three purposes: deflecting and slowing entering traffic, preventing wrong-way movements, and providing a mid-crossing refuge for pedestrians.
Circulatory Roadway: The active travel lanes around the central island.

Design Principles and Speed Control

The most critical principle of roundabout design is speed control, achieved through physical geometry rather than speed limit signs.

Interchange Forms

Mon, 04 May 2026 00:00:00 +0000

Interchange Forms

An interchange is a system of interconnecting roadways (ramps) in conjunction with one or more grade separations (bridges) that provides for the movement of traffic between two or more roadways on different levels. Interchanges are used to eliminate at-grade conflicts, maximize capacity, and improve safety on high-speed roadways.

Common Interchange Types

Interchanges are classified into several standard configurations based on their layout and operational characteristics.

1. Diamond Interchange

The diamond interchange is the simplest and most common type, typically used where a major freeway intersects a minor arterial or collector road.

Ramp Geometry

Mon, 04 May 2026 00:00:00 +0000

Ramp Geometry

Ramps are the connecting roadways that facilitate vehicle transfers between intersecting facilities at interchanges. Designing ramp geometry requires determining appropriate design speeds, horizontal curve radii, super-elevation rates, and vertical grades to ensure that vehicles can transition safely from one highway speed to another.

Design Speed of Ramps

Ramp design speeds are determined as a percentage of the design speed of the intersecting freeway, according to AASHTO Green Book Table 10-1. There are three design speed ranges:

Freeway Merge and Exit Design

Mon, 04 May 2026 00:00:00 +0000

Freeway Merge and Exit Design

Freeway merge and exit terminals are critical locations on a highway network where traffic transitions between high-speed through lanes and lower-speed ramp terminals. The design of these segments focuses on providing adequate length for vehicles to accelerate or decelerate safely, and providing clear, predictable geometric paths (gores) to minimize conflicts.

Acceleration and Deceleration Lanes

Auxiliary lanes are added at ramp connections to facilitate safe merges and exits: