With or Without Turn Lanes, There are 12 Things Going On
The problem with congested intersections is that — follow me closely here — they generally have too much traffic.
Granted, there are two other reasons intersections become congested, actually, there are three: 1) too much traffic (mentioned above), 2) not enough lanes, 3) or outdated intersection control (i.e., stop signs vs. traffic signals vs. roundabouts vs. interchanges).
Now if the problem is too much traffic, solutions consist of, 1) making an alternate route more appealing, 2) reducing traffic by working from home, providing public transportation, or car-pooling (whatever happened to car-pooling anyway? Find out next week!).
But if there are simply not enough lanes, solutions include, 1) adding lanes and, well, that’s it, adding lanes. And typically, the most effective lane to add is a turn lane. Either a dedicated left or a dedicated right turn lane improves the capacity of an intersection. And the length of the turn lane is important — it must be long enough to allow cars to stack outside of the through lane. Traffic engineering software models intersection traffic volumes and determines ideal geometry including the location and length of turn lanes — we call this, Traffic Engineering Magic (TEM).
The process involves collecting 24-hr traffic data to determine the overall peak-hour traffic volumes including the individual turning volumes for each direction (reference above graphic). Trucks are counted separately because let’s face it, they take longer to get moving and do whatever trucks do — computer models account for this anomaly.
Then, based on intersection geometry, turn lanes (or lack thereof), and existing traffic control (stop signs, signals, roundabout, etc.) the intersection is analyzed and various improvements tested to determine the most appropriate short-term/low-cost improvements. Often, future traffic volumes are also generated to analyze the long term effectiveness of any proposed improvements and to anticipate potential improvements for the future.
I believe it was Shakespeare, or possibly Johny Carson, who first observed, where you have a lot of cars, you have a lot of traffic. I could not have phrased it better myself.
Chet Skwarcan (traffic engineer, author, unique insights) with over 25 years of traffic engineering experience — online ideas available at TrafficEngineering.com/Services.