Tuesday, December 22, 2009

Teaching #1: The Late Merge Awareness Campaign

One might think that Lesson #1 should be the most important lesson.  I would agree.

I have 2 routes that I take home from work at night depending on what my MNDOT traffic map shows for congestion.  One route is on I-94, and the other route is 35W to Hwy 36.  The latter is usually referred to as my "social teaching route". It is inevitable that on this latter route that I put this lesson #1 into practice.  When you shake your fist at me while I speed by in the lane to be closed a 1/2 mile ahead, I'm actually making everyone's trip shorter.  Seriously, it is only about efficiency.  I wouldn't do it unless there was quantitative research behind it.  Here we go.

The debate between “early” or “late” merge seems more appropriate for human behavioral studies of aggressiveness, impatience, self-indulgence, and simple preference. However, there is general agreement in the engineering community on which method works best for each of two scenarios. 

Scenario 1 is when two lanes on the freeway are merging and the traffic density is light prior to, and at, the merge neck.  Additionally, speeds are at the posted limit throughout the pre-merge region.  This is not the category that I am talking about.  This is the 11 AM section of freeway with no traffic on it.  In this scenario, the early merge is most efficient.

I am addressing a second category, that is bumper-to-bumper/stop-n-go traffic for a mile before the merge neck, where the late merge is most efficient.  This is the 5 PM section of freeway where Hwy 280, Hwy 35W, and Hwy 36 all merge and it takes 5 minutes to travel 1/4 mile.  In this scenario the late merge is most efficient for the group as a whole.  The concept of delaying a merge when two lanes are funneled into one, caught on with traffic engineers after studies showed that traffic flow sped up as much as 15% over the old Minnesota “merge early and politely” philosophy.

I won't present all the studies validating this approach, but I will lay out a couple key components of why it is more efficient to merge late in heavy traffic scenarios.  First of all, why would the highway department put a merge "neck" (the spot where 2 lanes finally turn into 1) 1/2 mile ahead if they wanted you to merge now?  If they wanted you to merge now, they would put the merge "neck" here.  Simple but true. 

The second, slightly more complicated reason is based on the "rice test" phenomenon.  In the first case, dry rice is poured all at once into a funnel. In the second case, the same amount is poured slowly. These trials generally conclude about a one-third time savings to empty the funnel via the second method.  Traffic similarly moves slower the more dense it is packed.  The denser the traffic, the smaller the safety cushion around each driver, and the more cautious (i.e., slower) the driver becomes.  Thus, the longer you can spread traffic out over an additional lane between point A and B, the faster traffic gets from A to B.  In a theoretical 1 mile stretch of road, if you have traffic in 3 lanes for 1/2 of the mile and then 2 lanes for the last 1/2 of the mile, it moves more efficiently than if you have traffic in 3 lanes for 1/4 of the mile and 2 lanes for 3/4 of the mile.  Thus, that lane that eventually ends, should be used as additional "bandwidth" until it ends.

A third reason is that a large percentage of drivers in the discontinuous lane brake rapidly to join the end of the forming queue in the lane to be continued – even if their lane is still free flowing. This early merging out of the discontinuous lane is another cause of unexpected braking that sets up perfect conditions for rear-end collisions. Frequently, a driver takes it upon him/herself to be a “traffic cop” and block the discontinued lane with his or her vehicle, preventing those “late” merging drivers from going around the single lane queue. Fifteen percent of drivers admitted to straddling lanes in order to block late merges in construction zones, according to a recent study conducted by the Minnesota Department of Transportation. These behaviors really piss people off and only further the chance of an accident.

The solution from the MN DOT has been a series of electronic signs at merge locations that sense the speed of traffic flow and instruct drivers to "use all lanes" and "merge here" depending on the flow of traffic. 

"Basically, we want drivers to know that under normal traffic speeds, they should try to merge early to avoid unsafe merging maneuvers; however, when traffic is congested, drivers should use both lanes all the way to the definite merge point," said Servatius. "We can't completely rid the roads from congestion in a workzone, but data from the study revealed this method shortened queue lengths by 35 percent and reduced lane changing conflicts," said Mittelstadt. "We also hope for a decline in crashes and aggressive driving behavior." 

Sure there are counter arguments about just waiting your turn.  I agree with those arguments, but the point is that everyone in line is pissed off because things are moving slow.  If everyone understood that lanes merge at a specific spot because that is where they were meant to merge, and everyone used that 3rd lane until it merged, everyone would move faster during rush hour, and people would be happier in the new system.  It is just the transition that hurts.  

Here is an extensive reference from the US Dept of Transportation regarding this topic if you want my references.  In addition, the MN DOT has conducted a study right here at home. I carry a few copies of these studies in my glove box in case I get someone who yells at me with their window down.  I kindly pass them the studies and recommendation from the MNDOT study.  Like I said, it's quantitative.


  1. Too many studies, just move to ND!

  2. Wow, you've put a lot of time and effort into justifying your departure from the "merge early and politely" method. I have always wondered if it was more efficient to use the "merge late and piss-people-off" method. I don't often use this latter method but if I do, I will feel totally and compltely justified and may even cite your references. However, this post, and the time it took you to think about, study, read, and write it, is going to be in the Top 5 reasons to live in MT. That and the real good people who make real good beer. You know what I'm talking about too ;-)

  3. This cuts Jose's commute from his apartment to mine in half--instead of 57 minutes, it takes him 31 minutes. If merging late has that effect in New York City, I think you can use it to go mach speed in Saint Paul.