Intersection and Interchange Design
Intersections are critical nodes in a highway network where vehicles, pedestrians, and cyclists interact. Their design significantly impacts the capacity, efficiency, and safety of the entire transportation system.
At-Grade Intersections
At-grade intersections are locations where two or more roads cross at the same elevation. The primary challenge is managing the numerous conflict points where vehicle paths cross, merge, or diverge.
Conflict Points
Channelization
The use of pavement markings, raised islands, or other physical barriers to direct traffic into specific, predictable paths. Channelization separates turning vehicles from through traffic and reduces the area of conflict.
Intersection & Interchange Explorer
Explore conflict points at an at-grade intersection and basic interchange configurations.
Major Crossing Conflicts
Traffic Signalization Basics
When traffic volumes are too high for STOP or YIELD signs to operate safely and efficiently, traffic signals are warranted.
Objectives of Traffic Signals
- Provide orderly movement of conflicting traffic flows.
- Increase intersection capacity.
- Reduce the frequency and severity of certain types of crashes (e.g., right-angle collisions).
- Provide for continuous or nearly continuous movement of traffic at a definite speed along a given route under favorable conditions (signal progression).
Cycle Length ()
The total time required for one complete sequence of signal indications (e.g., from the start of green for one phase to the start of green for the same phase in the next sequence).
Webster's Optimum Cycle Length Formula
Where is total lost time per cycle and is the sum of critical flow ratios.
Rotary (Roundabout) Intersections
A rotary intersection is a specialized form of at-grade intersection where traffic circulates around a central island in one direction (usually counter-clockwise). All entering traffic must yield to the circulating traffic.
Weaving Action
The capacity of a rotary is highly dependent on the Weaving Length (the distance between an entry and the next exit point) and the width of the weaving section. The empirical formula for the practical capacity (, in PCU/hour) of a weaving section is:
Where:
- = width of weaving section (m)
- = average entry width (m)
- = length of weaving section (m)
- = proportion of weaving traffic (ratio of weaving vehicles to total vehicles)
Grade-Separated Interchanges
When intersecting roads carry high volumes of traffic, or when full control of access is required (as on freeways), at-grade intersections are replaced by interchanges. Interchanges use grade separation (bridges or tunnels) to allow traffic on crossing roads to flow continuously without interruption.
Diamond Interchange
Cloverleaf Interchange
Directional Interchange
Intersection Sight Distance (ISD)
Drivers approaching an intersection need clear lines of sight to see conflicting traffic and make safe crossing or turning decisions.
Sight Triangles
Important
If an adequate sight triangle cannot be physically provided due to existing buildings or terrain, engineers must implement more restrictive traffic control, such as lowering the speed limit, adding multi-way STOP signs, or installing a traffic signal.
Key Takeaways
- Intersections are the most complex points in a highway network due to the high density of conflict points.
- Channelization using islands and markings helps organize traffic and reduce the area of these conflicts.
- Traffic signals increase capacity but can also increase delay if improperly timed.
- Webster's formula seeks the cycle length that minimizes total intersection delay.
- Rotaries convert dangerous high-speed crossing conflicts into safer, low-speed weaving conflicts.
- Capacity is dictated by the dimensions of the weaving section and the proportion of traffic that must weave.
- Intersections are the primary source of delay and crashes in a road network due to numerous conflict points.
- Traffic signals increase capacity and improve safety if warranted, but can increase delay if improperly timed.
- Grade-separated interchanges eliminate crossing conflicts and provide uninterrupted flow, essential for high-speed, high-volume freeways.
- Webster's formula provides a theoretical minimum delay cycle length based on lost time and critical volume-to-capacity ratios.
- Clear sight triangles must be maintained at all intersections to ensure drivers can see conflicting traffic before committing to a maneuver.