Guest Post by Bryant Ficek, PE, PTOE, Vice President at Spack Consulting.
For many of our studies, stop signs, traffic signals, and roundabouts supply the necessary capacity and acceptable operations. Sometimes, though, we handle projects that require innovative thinking and lead us toward consideration of alternative intersections. Alternative in this case being defined as not traditional (the aforementioned stop sign, traffic signal, or roundabout) in terms of geometry, traffic control, or both.
The key to using alternative intersections, or conventional ones for that matter, is understanding the basic issues trying to be solved.
- Do you have a capacity problem, with the whole intersection or with specific movements?
- Are there a lack of gaps for turning traffic?
- Is there a safety issue at the intersection or on an approach?
- How’s the sight distance on each approach and lane?
- Do you have right-of-way limitations?
Once you understand the basic issues and concerns, you can then determine whether a conventional intersection will provide the answer or if you need to explore alternative intersections. Here are some those alternatives that we have considered for our projects and sometimes recommended:
- Access Management (right-in/right-out only or 3/4-access). Restricting one or more movements, either permanently or time-based.
- Offset “T” Intersection. Separating a four-legged intersection into two “T” intersections.
- Indirect Left Turns. Shifting the left turn movements on one or more approaches to a location several hundred feet away from the primary intersection.
- Split Intersection. Separating the main road traffic directions to create a pair of one-way intersections, similar to interchange intersections although without a freeway.
- Continuous Flow Intersection or Displaced Left-Turn. Removing the left-turning movements from the primary intersection and directing them to a separate roadway running parallel to the main lines.
- Echelon Intersection. Splits the intersection into two levels, creating a pair of one-way street intersections essentially on top of each other.
- Hamburger or Through-About Intersection. A variation of a signalized roundabout with the mainline through movements allowed straight through the intersection and the side-street/turning movements occurring on a circulatory roadway around the intersection. (Image is of a Hamburger Intersection in Fairfax, VA)
- Synchronized Split-Phasing or Double-Crossover Intersection. The through and left turn movements on the mainline cross over to the opposite site before the primary intersection, similar to a Diverging Diamond Interchange.
A simple internet search will provide lots more detailed information on these and other types of designs to accommodate traffic. As mentioned, always keep in mind that your solution(s) should be trying to solve the problem. There’s no need to be fancy if it’s not needed.
Intersection of Woodward Ave and Eight Mile Road, Detroit: Three levels. Through traffic on top, on the bottom, and a middle level with turning functions. This intersection used to handle 240,000 vehicles per-day back in the mid-1970s:
A similar design is used at the junction of Woodward Ave and I-696. Woodward through traffic is below, I-696 is in the middle, and Woodward turning functions are on-top:
Michigan U-Turn: No left turns allowed on boulevards. Take a median U-Turn and return for a right turn instead. Example: Intersection of Woodward Ave and Square Lake Rd.
One form of New Jersey “jug-handle” intersection, all turns from jug-handle rather than from the through portion of the major roadway. Shown: Intersection of Truck US 1 & 9 and Hackensack Ave, Kearney, NJ, just east of I-95 Exit 15E.
Another form of New Jersey “jug-handle” intersection, US-202, south of Flemington, NJ. No left turns allowed, all left turns from jug-handle and U-Turn.
Traffic circle, junction of US-202, NJ Rt 12, and NJ Rt. 31, Flemington, NJ:
Traffic circle: MA Hwy 2, MA Hwy 2A, Commonwealth Ave, and Barrett’s Mill Rd, Concord, MA Hwy 2 is a freeway entering the circle from the west.
One of these freeway intersections, I don’t even know what they are called. C-470 at Morrison Rd, Morrison, CO. They work decently when the ramps are below the freeway and there is plenty of turning room, but they don’t work nearly as well is the ramps are above the freeway and tight, as is the case at the interchange of Santa Fe Dr (US 85) and Evans Ave in Denver. Another similar interchange with the ramps below the freeway is the interchange of I-25 and Garden of the Gods Rd in Colorado Springs.
Finally, a Diverging Diamond interchange, in North Carolina at I-40 and NC Hwy 123. Once the locals get used to driving on the wrong side of the road these work pretty well to eliminate left turn signals and increase the through traffic signal time. This source is a video about it.
News item on Diverging Diamond interchanges shows that they can move quite a bit of traffic:
PS: I am having some trouble trying to verify my email and WordPress account, as I have two different free WordPress accounts associated with my private email address, and WordPress does not want to let me add my University of Colorado email address to either of my existing free accounts. I made a couple of posts on your Strict Speed Limit topic a few days ago.
My University of Colorado email address is: email@example.com. I am a 3rd year graduate urban & regional planning student (MURP) at the University of Colorado – Denver specializing in Regional Sustainability, on top of a 30-year career in trucking and wholesale fresh food supply chain, warehousing, and distribution logistics.
I’m assuming a “3/4 Intersection” is a place where left-ins are allowed but left-outs are prohibited? I’ve been calling those RILIROs (Right-In, Left-In, Right-Out), mainly because “riley-row” is kind of fun to say, but I might switch to 3/4-Intersection after seeing that. I’d appreciate hearing what others use.
Riley-Row does have a nice ring to it, but the industry standard jargon is 3/4 Intersection (disclaimer – it has been in all of the jurisdictions I’ve worked and I believe that is the standard term). Mike