In a previous post we explained one type of traffic bottleneck. In this post, we attempt to explain another type of bottleneck, and explain why it is not really practical to “fix it.”
Most intersections/roadway segments/highways operate with little delay during most of the day. Long delays and queues occur when too many vehicles show up to the same spot in a short amount of time.
This generally happens from Monday to Friday, usually sometime between 7:00 AM and 9:00 AM in the morning and again in the afternoon between 4:30 PM and 6:30 PM.
Most of our infrastructure is design to minimize delays for these periods – at most 20 hours of a 168-hour week.
How we do we “fix” this situation? We make the facility larger (we add capacity). Like our last post, we can use water again as a metaphor by which to explain this phenomenon (and again accompanied by an animation).
At the beginning of the animation, water is flowing through the funnel without any issue (1). Suddenly, the flow of water increases significantly for a short period time (2) and the funnel fills to near its top (3). To accommodate this additional flow of water, the funnel is expanded (4), and the short-term water flow increase can be accommodated by the funnel (5).
Again, let’s explain this situation in terms of cars through an intersection (all changed words are bolded):
At the beginning of the animation, vehicles are flowing through the intersection without any issue (1). Suddenly, the flow of vehicles increases significantly for a short period time (2) and the intersection has delays and queuing (3). To accommodate this additional flow of vehicles, the intersection is expanded (4), and the short-term vehicle flow increase can be accommodated by the intersection (5).
This is great for that intersection, but of course vehicles do not only travel through one intersection on their journey. There’s an entire system of intersections (or funnels) to travel through from the beginning of a journey to its end.
If a funnel occurring upstream of (before) another funnel is expanded to allow flow, then downstream funnels now see water levels rising.
Let’s say Funnel B was the funnel we just expanded – flow was getting heavier, so the funnel was widened? Did we consider whether Funnel C could accommodate the additional flow that would arrive? There might not even be additional demand for Funnel C – could be the same flow, just previously constrained by Funnel B.
What if Funnel A can longer handle its flow? By the same standard we just set, Funnel B needs to be upgraded again. Do we consider the impacts on Funnel C? What about Funnel D, that works fine? Does it still “work?”
It’s not a hard thought process to consider these roads (think of Funnel A as the interstate, Funnel B as arterials connecting to the interstate, Funnel C and D as collectors or neighborhood roads connecting to the arterials).
We’ve done enough transportation analysis to know that very rarely do engineers working on Project A consider the impacts on B, much less C and D. These secondary impacts are very real, and this helps explain why we can never really “fix” bottlenecks. Typically, we just move them to a new location, and we end up chasing bottlenecks all around the area in which we are working.