Special intersections — Idaho Permit Test

A thru-turn is a way of managing intersection traffic by prohibiting direct left turns.

The left turn across oncoming traffic is the movement that causes the worst crashes, so this design removes it.

A thru-turn is a method of managing intersection traffic by prohibiting direct left turns

To turn left at a thru-turn intersection you continue through it, make a U-turn at the next signal, head back and make a right turn.

The turn is made in two easy movements instead of one difficult one, which is the whole point of the design.

continue through the intersection, perform a U-turn downstream at the next traffic signal from the traffic signal/intersection you just went through, as directed by lane markings, head back toward the intersection, and make a right turn.

A single point urban interchange gets its name from the traffic signals that control turns from a single point at the center of the interchange.

All the turning movements are gathered into one signal, so there is one place to watch rather than four.

gets its name from the traffic signals that control turns from a “single point” at the center of the interchange

At a single point urban interchange, stop at the stop lines so you are not too far out in the intersection where you might impede cross-traffic.

The intersection is larger than it looks, and a car stopped past the line is standing in somebody's turn.

Make sure to stop at the stop lines so you won't be too far out in the intersection where you might impede cross-traffic

If you leave a freeway by mistake at a single point urban interchange, you cannot travel straight across the highway and get on at the other side; you turn left or right onto the intersecting highway and find a safe place to turn around.

There is no straight-through movement for a driver who has made a mistake, and trying to find one is how a car ends up going the wrong way.

you cannot travel straight across the highway and get on at the other side

A Diverging Diamond Interchange temporarily channels traffic lanes to the left side of the highway in order to create direct access with the interstate.

Driving on the left feels wrong, which is why the design is signed and marked as heavily as it is.

temporarily channels traffic lanes to the left side of the highway in order to create direct access with the interstate

At a diverging diamond interchange, opposing traffic crosses at the signals on each side of the bridge or overpass.

The crossing happens at a signal rather than at speed, which is what makes the switch of sides safe.

Drivers will notice that opposing traffic crosses at the signals on each side of the bridge/overpass

Motorists at a diverging diamond interchange must stop at the stop lines so they are not out in the intersection where they could impede cross-traffic, pedestrians or bicyclists.

People on foot and on bikes cross the middle of this interchange, and a car stopped past the line is standing in their path.

Motorists will need to stop at the stop lines to ensure they are not out in the intersection where they could impede cross-traffic, pedestrians, or bicyclists

Turning right off the interstate at a single point urban interchange means staying in the right lane, yielding, turning right, merging onto the highway and watching for pedestrians.

The merge and the crosswalk happen at the same moment, which is why the pedestrian check is written into the movement.

Yield, turn right, and merge onto the highway. Watch for pedestrians

Bicyclists and pedestrians at a single point urban interchange use the signalized pedestrian crossing and are then directed to a center pedestrian island in the middle of the interchange.

They cross in two stages with somewhere to wait in between, which is where a driver will find them standing.

then are directed to the center pedestrian island in the middle of the interchange

Going straight over a diverging diamond interchange means proceeding through the signal at the entrance, following your lane to the opposite side of the overpass, then through the second signal and back to the right side of the road.

The lane does the work. A driver who follows the markings ends up back on the right without deciding anything.

they will proceed through the traffic signal at the entrance to the interchange and follow their lane to the opposite side of the overpass

What does a thru-turn intersection prohibit?

Direct left turns. The left turn across oncoming traffic is the movement that causes the worst crashes, so this design removes it. July 2026 Idaho Driver's Handbook, p.63: “A thru-turn is a method of managing intersection traffic by prohibiting direct left turns”