Complex Road Designs

Complex road designs are fun!

In the Hacking Traffic Flow lessons, we talked about strategies for dealing with freeway-like features. Here we will explore several types of interchanges we might encounter and how to handle them.

We’ll also take a look at how roundabouts work, and learn to love them.


Topic 13: Complex Road Designs

Topic Introduction

Now that we’ve covered the basic components, let’s take a look at some road, designs you might encounter.

How Roundabouts Work

First, Roundabouts.

Traffic circles and roundabouts are a series of one-way by one-way intersections. The modern roundabout is designed to reduce speeds and conflicts to improve safety for all users.

Entering drivers yield to traffic that is already in the roundabout. Always enter a roundabout in a lane control position and maintain that control until you exit. Stay far enough left to keep clear of exits you are not using. The design reduces the turning radius for entering and exiting traffic, increasing right hook and drive-out potential if you don’t control your lane.

Stay alert for entering vehicles and don’t assume they will yield. Many Americans do not know the rules for roundabouts.

You’ll find it surprisingly easy to ride around most roundabouts if you use lane control. Most roundabouts keep motorists to bicycle speed, anyway.

Two-Lane Roundabouts

Two-lane roundabouts come in different configurations. Some have two lanes all around the circle, others have two lanes for only part of the circle and then drop to one lane. There is usually a sign posted on the approach, to alert traffic to the required lanes to use for various movements.

Most modern roundabouts do not require a lane change once inside the roundabout. Drivers using it to turn left are required to enter it in the left lane and leave it from the left lane. Entering drivers must to yield to traffic in both lanes.

Traditional Interchanges

The most complex road conditions we face as cyclists involve interchanges and the freeway-like features they sometimes introduce onto the streets we use.

Interchanges, in which one road goes over the other, come in a variety of configurations. While they are usually used to get a regular street over or under a freeway, some are at the intersection of two regular streets. Some are easier to negotiate than others.

Traditionally, there were two basic types: the diamond and the cloverleaf. Many interchanges are a combination of diamond and cloverleaf geometries.

With the diamond, the ramps connecting the two roads form regular intersections, either controlled by signals or by stop or yield signs. Such intersections are fairly simple for bicyclists to navigate, as they don’t require any high-speed merging maneuvers.

In areas with heavy traffic flow into the interchange, the surface street may have one or more diverging lanes. Look for overhead signs to help you determine if this is the case.

In the cloverleaf, the ramps circle around and meet the cross roads in parallel, meaning drivers must usually change lanes to access the new road.

“Weave lanes” are characteristic of cloverleaf interchanges. Motorists will be merging in and out of the right through lane from the weave lane.

This can be quite turbulent. It’s best to maintain a position in the left-tire track of the through lane. This makes you most visible to entering motorists and encourages the exiting ones to wait and change lanes behind you. Even if the weave lane seems long, do not move into it!

Single Point Urban Interchange (SPUI)

In recent years, some new types of interchanges have been invented and built. The first is the “Single Point Urban Interchange,” or SPUI.

With a SPUI, the lanes for turning left from the freeway onto the surface street meet at a single point in the center. The lanes for right-turning traffic merge and diverge with the surface street.

The one special concern for bicyclists is the distance across the intersection. Some SPUIs are more than twice the length of a suburban intersection. A bicyclist entering the intersection on a yellow light may not get through before the cross traffic starts. Look for a safe place to pull over and wait if this happens.

When a SPUI is used at a freeway crossing, the only street used by bicyclists is the one with the traffic signals. If both roads served by the SPUI are open to bicyclists, you have a choice when crossing on the street with the free-flowing traffic.

In this example, you can ride over the overpass along with other through traffic. This requires a lane change out of the lane that diverges to the intersection before climbing the bridge.

In these light traffic conditions, this an easy and fairly comfortable ride. It would be less so in heavy traffic. In some situations you might have to change lanes to the right on the other side of the bridge. Here, the rightmost lane will end, so we stay in the #3 lane.

For a less complicated crossing, you can use the ramps to exit the overpass, go through a signalized intersection and then re-enter on the other side.

In this case, it took a little longer to go through the intersection. The wait time at the red light was two minutes and 30 seconds, but we didn’t have to climb a hill. The net time savings using the overpass was one minute. We were riding 50-pound cargo bikes, so your mileage may vary.

Diverging Diamond

One of the more innovative new designs is the diverging diamond.

The diverging diamond interchange may look very strange and intimidating, but it is fairly easy to negotiate. Cyclists have only to be concerned with diverging right turns if the street has a bike lane. The other crossings are fully signalized and are fairly short, so you needn’t worry about getting stranded, as you might in a SPUI.

Frontage Roads

Also related to freeways are frontage roads. These are lower-speed one-way roads parallel to freeways. They tend to make for good cycling, because the ramps are on the left side. If you control your lane, the only drivers you need to worry about are the ones coming from your right.

Riding on Freeways

In some states we’re allowed to ride on the paved shoulders of interstates and other freeways. Crossing the exit and entrance ramps can be treacherous, and may be illegal. Either way, it is best to exit and re-enter the highway.

Lesson Conclusion

In the final two lessons of the Mastery Course, we will put our roadway and traffic flow knowledge to use in creating strategies for stress-free cycling, and planning ahead for the best routes through our cities and suburbs.

In the next lesson, we will look at the many types of bicycle-specific infrastructure.


Animation of how to drive through a roundabout

Animation of how a Single Point Urban Interchange works. Here’s a video of savvy cyclists using the SPUI in the animation

Animation of how a Diverging Diamond Interchange works

Some CyclingSavvy videos:

Clearwater Beach Roundabout: Odd configuration requires a lane change in the circle

Northwest Highway at Rush Hour: This looked seriously intimidating, but with savvy cycling principles turned out to be a piece of cake

Ivanhoe Interchange Westbound: Now this interchange is history, as the Florida Department of Transportation has redesigned and rebuilt it. But this was one of the most intimidating urban interchanges in Orlando (and probably anywhere)

U.S. 17-92 at I-4 Interchange: Cyclists must use this because there are no alternatives