There has been a lot of discussion about the number of travel lanes needed on 3rd Avenue South lately (click here, or here). The Transportation and Public Works committee was presented with a four-lane design for the section south of 8th Street last week, and sent the matter on to City Council with “no recommendation.” Since then, I’ve been asked by a couple of people to help translate the traffic studies performed for the corridor. I am a Professional Engineer with nearly a decade of experience in traffic engineering, and a really fancy PTOE certification to go with it. I thought I would share what I’ve summarized for others here on streets.mn. Because the objections to the three-lane section appear to be based on concerns about traffic, this post is entirely about traffic. The case for people traveling by foot or on bicycle, or which configuration is more aesthetically pleasing has already been well articulated by many.
The three-lane configuration is the better alternative for motor vehicles. It is (arguably) better for vehicles operationally and definitely better from a safety standpoint. This is not a bikes vs. cars scenario; three lanes are better for cars.
Here’s the long version:
There were two traffic studies performed by the City of Minneapolis Traffic Engineering consultant: one in June 2015 that provided a comparison of the four-lane and three-lane configurations, and one in January 2016 that provided further analysis of the (then preferred) three-lane section. Because the study in June presents the two options side-by-side, my summary is based on those results. The results of the January study are similar, but it only compares the three-lane section to existing conditions. Since it’s clear that something is going to get built, we’ll look at the comparison between the two alternatives.
Both options result in a near “free-flow” condition for the segment south of 8th Street South, with overall intersection of “LOS C” or better. The concept of level of service (LOS) is a conversion from overall intersection delay in seconds per vehicle to a letter grade system from A to F, with A meaning no delay and F meaning significant delay. The thresholds for a change from one LOS to the next are somewhat arbitrary. For example, a signalized intersection with overall delay of 19.9 seconds would be reported as LOS B, while an intersection with 20.1 seconds of delay would be reported as LOS C. When LOS changes between alternatives, it’s important to look at the overall intersection delay to see if the alternative scenarios are on the threshold between two LOS grades. In this example case, as a traffic engineer, I would consider the two scenarios to be operationally equal.
After looking at results in Table 2 of the June 2015 traffic study, there is almost no difference between the two alternatives. AM and PM Peak Hour differences in intersection delay are shown here:
Both of these alternative result in perfectly acceptable traffic operations. For a downtown area, these traffic operations are actually surprisingly good. The three-lane configuration has slightly lower delays during the AM Peak Hour, while the four-lane configuration has slightly lower delays during the PM Peak Hour. These numbers are so close, that it’s probably a wash, but the three-lane configuration does net slightly better operations. This may seem counter-intuitive, given that the four-lane configuration has more through lanes than the three-lane configuration, right? Here’s why:
Why Traffic Engineers Love Left-Turn Lanes
The benefit of the center left-turn lane can best be seen on 3rd Avenue in the blocks between 9th and 11th. In the three-lane configuration, vehicles waiting to turn left on 9th or 10th queue in a turn lane in the middle of the roadway.
However, in the four-lane configuration, there is no space for left-turn storage. In this case, vehicles waiting to turn left on 9th or 10th queue in a through lane. Even though two northbound vehicles can go through at the same time at 10th, one runs the risk of being stopped behind a left-turning vehicle.
Regular drivers of the roadway will tend to stay right so as to avoid this trap, and will not fully utilize the additional through lane. For this reason, two through lanes do not really provide twice as much capacity as one. For examples of drivers avoiding the left lane during congested periods because they’re worried they’ll get trapped behind a left turner, I invite you to spend some time on Broadway Ave in Northeast Minneapolis.
The addition of left-turn storage has also shown to reduce vehicle-to-vehicle crashes. Through vehicles stuck behind a left-turning vehicle will take risks to get into the right lane, resulting in side-swipe crashes. Or, the stopped vehicle in the through lane can come as a surprise, resulting in rear-end crashes. The four-lane to three-lane conversion is popular with many traffic engineers because of the safety benefit of the addition of this left-turn storage.
Improving safety without impacting operations? This is a traffic engineer’s dream. There’s not even a trade-off to be made! What’s especially interesting on 3rd Avenue South is that the existing configuration consists of two through lanes in each direction and left-turn lanes where needed. So, the argument here is really about what is being taken away: a through lane in each direction, or a left-turn lane. Given that the operations between these two alternatives are practically the same, the safer option should prevail.
But Why is 3rd Avenue So Backed Up Today?
In short, because of the intersection of 3rd Avenue and 5th Street. The January 2016 traffic study recommends additional LRT detection at this intersection, such that, when trains are not present, more green time can be assigned to 3rd Avenue. The conclusions of the January 2016 traffic study state:
The 3rd Ave/5th St intersection is expected to be the most problematic location. The proposed mitigation measures are expected to improve the operations of this intersection during both the AM and PM peak hours. This conclusion is based upon the reduction in overall intersection delay estimated and the improved southbound travel times (reduction in vehicle queues) over existing conditions [emphasis added].”
The benefits of this detection are, in my opinion, even better than what can be captured in SimTraffic simulation. Regardless of which alternative is chosen, traffic on 3rd Avenue may actually flow better than it is today with this improvement at 5th Street.
What About Future Traffic?
The traffic studies used a 0.5% per year growth rate to increase hourly traffic volumes collected in 2011 for use as 2016 volumes in the analysis. The January study also includes the following helpful charts, showing that Average Daily Traffic (ADT) volumes on Third Avenue around 8th Street have actually decreased during that period (blue dashed line), while volumes around 3rd Street have remained flat (blue dashed line again). The estimated Average Daily Traffic (ADT) is represented by the red dot.
The intersection delays I’ve discussed above are all based on a conservative estimate of the traffic that is out there today.
A Note About Vehicle Level of Service
I know streets.mn readers are pretty savvy with respect to transportation jargon, but just in case here’s what you need to know about Level of Service. The 2010 edition of the Highway Capacity Manual (HCM) contains the equations and guidance that traffic engineers use to calculate LOS. Here are the delay thresholds:
It also includes this guidance:
“Analysts and decision-makers should always be mindful that neither LOS [Level of Service] or any other single performance measure tells the full story of roadway performance” (Page 8-11).
The HCM does NOT provide guidance on what level of service roadway agencies should strive for, although generally speaking, LOS D or better is considered “acceptable” in most jurisdictions. Exceptions are large cities and metropolitan areas, which may consider LOS E acceptable, or small towns where drivers are not accustomed to long delays, where LOS C or better may be more appropriate. The HCM also states:
“The existence of a LOS F condition does not, by itself, indicate that action must be taken to correct the condition.”
So, even in a municipality that has stated that they strive for LOS D or better, LOS E or LOS F conditions often exist. At some point, it becomes a value judgement for the decision-makers.