Interchanges use overpasses or underpasses to eliminate at-grade conflicts between crossing roadways. They are essential for freeways and high-speed expressways.

An intersection is the general area where two or more highways or streets join or cross. Because vehicles moving in different directions must share the same physical space, intersections are the most critical, complex, and dangerous components of a highway network.

The efficiency, safety, average speed, operating costs, and overall capacity of a corridor are largely dictated by the design and control of its intersections.

Intersections are broadly classified by how they handle conflicting traffic flows in vertical space.

A fundamental concept in intersection safety and control is the Type II Dilemma Zone. When a traffic signal turns from green to yellow, an approaching driver must make a split-second decision: brake hard to stop before the stop line, or maintain speed to clear the intersection before the light turns red.

Depending on the vehicle's speed ($v$), the perception-reaction time of the driver ($t_r$), the safe deceleration rate ($a$), and the width of the intersection ($w$), there may exist a physical zone where neither option is safely possible.

If a driver is in the dilemma zone when the light turns yellow:

Traffic engineers eliminate the dilemma zone by precisely calculating the Yellow Change Interval and the All-Red Clearance Interval.

Where $Y$ is the required yellow time. This ensures that a vehicle that cannot safely stop has exactly enough yellow time to reach the stop line before red. The subsequent all-red interval provides the time to physically cross the wide intersection block.

A fundamental concept in intersection safety is the conflict point. This is any specific location within the intersection area where the paths of two vehicles (or a vehicle and a pedestrian) cross, merge, or diverge.

A primary goal of intersection design and control is to reduce the number and severity of these conflict points.

Traffic engineers apply different levels of control based on traffic volumes, speeds, sight distance, and crash history. The goal is to apply the minimum level of control necessary for safety and efficiency.

Traffic signals should not be installed arbitrarily, as unwarranted signals can increase delays and rear-end collisions. The Manual on Uniform Traffic Control Devices (MUTCD) provides 9 specific Warrants—a set of minimum conditions. If none are met, a signal should not be installed.

Key Warrants include:

Accurate intersection analysis requires detailed Turning Movement Counts (TMCs), tracking the specific number of left, through, and right-turning vehicles per approach.

Furthermore, Heavy Vehicle Adjustment Factors are crucial. A truck or bus takes up more physical space and accelerates much slower than a passenger car. In capacity analysis, heavy vehicles are often converted into "Passenger Car Equivalents" (PCEs) to standardize the flow rate calculation.

Modern intersection control in North America relies almost universally on the NEMA (National Electrical Manufacturers Association) Dual-Ring architecture.

A modern roundabout is a specific type of circular intersection distinct from older, larger traffic circles or rotaries.

Channelization is the strategic separation of conflicting traffic movements into distinct, definite paths of travel using traffic islands, raised medians, or painted pavement markings.

Drivers approaching an intersection must have an unobstructed view of the intersecting roadway. This clear line of sight is known as the sight triangle.

For a stop-controlled intersection, the clear sight distance ($ISD$) required along the major road is calculated based on design speed ($V$) and critical gap ($t_g$):