Explaining Bottlenecks

January 18, 2018 by Dan Hennessey

Hey! Why Are We Stopped?

We all get in caught in traffic, sometimes in strange places where we least expect it.

When a freeway goes from three lanes to two, it’s fairly obvious what happened.  When there is a signal after a long stretch of open road, usually we understand that too.  Most of the time when friends and family ask why they get slowed down, I can explain why to them.

There’s one situation that has always been hard to explain.  In the traffic engineering lingo, it would be called a “temporary capacity decrease.”  Most people would call it a lane blockage or closure – this can be due to a collision, to short-term construction work, to an animal slowly crossing the road, even to something happening on the shoulder or off to the side of the road.

Often, when the event that causes the back-up occurs, the people it affects the most cannot see it.  And when the blockage has been cleared, its effects can still be felt for many minutes and many miles.

People will say to me, “I never saw an accident” or “All of a sudden we were moving again.”  The animation below will help explain this situation.

Sure, it’s a picture of water in a funnel.  It’s the same premise (and actually, a lot of traffic flow theory is based on the motion of water).

At the beginning of the animation, water is flowing through the funnel without any issue (1).  Suddenly, a blockage occurs for a short period time (2) and the funnel fills to near its top (3).  During this time water is added to the funnel, but due to its distance from the blockage, it cannot “see” what the problem is.  The blockage is then removed (4), and the water at the top of the funnel never saw what happened.

Water begins flowing again (5), and the volume of water in the funnel decreases (6).  The water that was at the top of the funnel begins moving faster, even though it’s a ways away from where the actual problem was.  By the time late-arriving water leaves the area of the blockage (7), there’s no evidence of a problem there.

Now let me write those paragraphs again talking about cars (all changed words are bolded):

At the beginning of the animation, vehicles are flowing through the roadway without any issue (1).  Suddenly, a blockage occurs for a short period time (2) and the roadway begins to queue vehicles backward (3).  During this time vehicles are added to the roadway, but due to their distance from the blockage, drivers cannot see what the problem is.  The blockage is then removed (4), and the vehicles at the back of the queue never saw what happened.

Vehicles begin flowing again (5), and queue of vehicles on the roadway decreases (6).  The vehicles that were at the back of the queue begin moving faster, even though they are a ways away from where the actual problem was.  By the time the late-arriving vehicles leave the area of the blockage (7), there’s no evidence of a problem there.

Next time you get stuck, remember that there’s no magic to why you are moving again – just the passage of time and space.

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Written by Dan Hennessey

Dan Hennessey

Dan Hennessey, P.E., PTOE is Director of Transportation Engineering at BIG RED DOG. Dan holds a Master’s Degree in Civil Engineering (Transportation) from the University of California, Berkeley and a Bachelor of Science in Civil Engineering from The Ohio State University. Dan is responsible for managing our Raving Fans and prospective clients, overseeing the performance of our traffic and transportation engineering design staff, steering our marketing strategy, and spearheading our community outreach and volunteer efforts. Dan’s professional experience and expertise focuses on Traffic Impact Analysis (TIA) studies, highway, freeway and arterial operations analysis, signal coordination and synchronization, traffic signal design, travel demand forecasting, and pedestrian and bicycle facility design.