Traffic & Safety

When a traffic signal is used, it operates in several steps called phases.

The left turn phase is especially important for safety. At busy intersections, a green arrow is usually needed to make left turns safe.

In a typical diamond interchange, the traffic signal goes through at least three phases:

1. Left turns from the interchange crossroad onto the entrance ramp.
2. Straight-through traffic on the crossroad in both directions.
3. Traffic exiting the ramps.

Conventional Ramp Intersection Left-turn Signal Phasing

Protected
Left Turn Movement

Through Movement (with possible Permissive Left Turn)

Exit Ramp
Movements

The ramp intersection traffic signals at a Diverging Diamond Interchange (DDI) work differently than most ramp intersections. Instead of the usual setup with three phases for each cycle at the ramp intersection traffic signal, a DDI shifts the traffic lanes to the opposite side of the road. This change allows for free right turns and left turns from the crossroad to the entrance ramps.

For example, when making a left turn onto a ramp, you do it before reaching the intersection, so you don’t have to wait for a green arrow or for oncoming traffic to pass.

Left turn onto a ramp before reaching the intersection

This setup simplifies the traffic signals with only two phases, helping cars move more smoothly in and out of the ramps. Although it might initially seem strange, clear signs, signals, and road markings will guide you safely through it.

Detailed information about how to drive through the Diverging Diamond Interchange is shown in the video at the top of this page.

Freeway Interchange Background

At a freeway interchange, a major highway crosses over or under a lower priority crossroad. The traffic getting on or off the freeway meets the crossroad at the ramp intersections

After looking at different interchange options, including how much traffic there is now and in the future, the cost to build, the need for land, how well traffic moves, and safety - a decision is made on which type to use. This process is usually explained in an Interchange Justification Report (IJR) and often includes more detailed environmental studies, usually called the National Environmental Policy Act (NEPA) process.

In Iowa, the most common interchange designs are diamond interchanges, cloverleafs (full and partial), and folded diamonds, which combine elements of both (see the diagrams below). These designs are effective in many locations. Depending on where they are, most of these interchanges can have traffic lights or roundabouts where the ramps meet the crossroad.

Conventional Interchange Types

Diamond Interchange
(with stop signs or signals)

Partial Cloverleaf Interchange

Folded Diamond Interchange

At some higher traffic volume and high growth locations, other innovative interchange designs have shown increased safety and reduced delay. The most common of these are diverging diamond interchanges (DDIs), roundabout interchanges, and single-point urban interchanges (SPUI).

Innovative Interchange Types

Diverging Diamond Interchange (DDI)
 

Roundabout Interchange

Single-Point Urban Interchange (SPUI)