The Unprotected Left (right) Turn

It is said the unprotected left turn (right turn in left-hand drive countries like Australia, but I will write as an American here) is hard for autonomous vehicles (AVs). (Even ignoring pedestrians, which magnify the complexity if there were to be treated as full-fledged users rather than an after-thought.)

With an unprotected left turn there is ambiguity about whether gaps between vehicles are large enough for the AV to squeeze through safely, and whether oncoming traffic will yield to an attempt to cross, particularly as the wait gets longer and longer and the passengers in the turning vehicle become more and more impatient.

It’s hard for humans too, left-turns comprise a quarter of all pedestrian crashes. So why do we have it? I.e. why don’t we protect the left turn.

It is  a matter of vehicle delay and storage space. On a one lane per direction road, or even a two lane per direction road, vehicles that are queued to turn could block vehicles that might otherwise go straight (or vice versa) when there is no turn bay and they don’t simultaneously have a green.  

We could have a phasing configuration which gave each approach (North, South, East, West (N, S, E, W)) its own green time.  In this  case, if flows were more or less equal between left turns and through/right movements, this might be the optimal solution. But if flows were dominated by one or the other, then it would be less than efficient. 

Alternatively, if we have turn bays (dedicated turn lanes) to keep vehicles out of each other’s way, we could have a configuration (N/S Through/Right, N/S Left, E/W Through/Right, E/W Left) which paired the turns. But turn bays use up lots of space that could be alternatively used for just about anything than temporarily storing cars.

And of course these could be mixed (N/S Through/Right, N/S Left, E, W) depending on relative flows.

But if the North flow > South or East flow > West (or vice versa), then these strategies will leave large gaps that could have been used by crossing traffic, but weren’t because the signal wasn’t timed for it.

With sufficient real-time information about flows, the signals could be adjusted to turn the lower flow approach to red when there are no vehicles approaching to protect the higher flow approach. This information requires knowing total approaches, but would be more accurate if the number on each turn (left, through, right) were also known, but this might be hard to discern simply by their location if the use of turn signals is imperfect, and there are too few dedicated lanes.

Update: We could prohibit left (right) turns. This is down in Moscow, so I understand. The left-turn ban at intersections is useful with low-rate flow turning left, assuming all left-turn vehicles are willing to do right turns several times to get to their destinations. But, this may impose a heavy burden of additional traffic on other road sections.

Or we could just have more roundabouts. These create other issues.

Porsche waiting to make a left turn, despite a presumably high value of time.

Deleting a road in Green Square

Green Square is developing rapidly
Green Square is developing rapidly

In New York City it was found that traffic flowed better after the diagonal Broadway was closed to traffic in a few places, including Times Square. Sometimes there are street segments that might have once made sense in an earlier era, but have hung around far longer than needed.

This suggested example is around the Green Square rail station and redevelopment site, (map) involving these same two roads I talked about in a previous post at a different location. While it is no Times Square, there is a massive amount of development going in.

Green Square is a major redevelopment site just to the east of the image. The Green Square rail station is the south Central area. The pedestrian environment in this area is deplorable.

Today the Pink Box is bisected by the end of O’Riordan Street (the pink line segment), which otherwise more-or-less continues to Wyndham Street on the West. Botany Road is the main north-south road on the East side. Bourke Road here is East-West through the image (though it is mostly a north-south road). As can be seen in the image, most traffic follows O’Riordan to Wyndham anyway, to the regret of local residents. That would not change.

In this proposal, the Pink Box would be an enlarged Pedestrian Plaza.

Green Square and O’Riordan Street.

The required change is simple: Close O’Riordan Street in front of Green Square Station.

(While we are at it, the Wyndham Street/Bourke Road intersection doesn’t need to flare out like that either).

The other streets are all two-lanes in each direction, but this diagonal makes signalling more difficult, and increases lost time, for very little gain (traffic from O’Riordan (NB) wanting to switch to Botany (NB) or Bourke (EB/NB) and vice versa).

All of these roads start near each other (all three: O’Riordan, Botany, and Bourke end at the airport in the south; O’Riordan (i.e. Wyndham/Gibbons) and Botany (i.e. Botany/Regent) come together in the North, and run into Circular Quay, while Elizabeth, which splits from Bourke terminates there as well. Bourke itself winds up about 10 blocks east. People who want to switch paths can use an East-West link (like Bourke here, and others up and down the corridor) as needed.

By simplifying the intersection, and retiming the signals I posit that both traffic flow and the pedestrian environment would improve. An appropriate set of pedestrian crosswalks at each leg of each intersection could be provided, and each crossing given a reasonable amount of green time. A pedestrian going from the northwest side of Wyndham to the Green Square station would only take two rather than 4 street crossings.

There is already planned a “Green Square to Ashmore Connector”, (south of this location) but the analysis of that assumes this leg stays in place. The additional capacity there is one more way for traffic to move east-west or to change north-south routes.

Panorama - Second Street from the right is the pink line segment on the map.
Panorama – Second street from the right is the pink line segment on the map.


Traffic Flow Variation and Network Structure

Recent working paper

Figure4This study defines and detects competitive and complementary links in a complex network and constructs theories illustrating how the variation of traffic flow is interconnected with network structure. To test the hypotheses, we extract a grid-like sub-network containing 140 traffic links from the Minneapolis – St. Paul highway system. We reveal a real-world traffic network comprises both competitive and complementary links, and there is a negative network dependency between a competitive link pair and a positive network dependency between a complementary link pair. We validate a robust linear relationship between standard deviation of flow in a link and its number of competitive links, its link correlation with competitive links, and its network dependency with both competitive and complementary links. The results indicate the number of competitive links in a traffic network is negatively correlated with the variation of traffic flow in congested regimes as drivers are able to take alternative paths. The results also signify that the more the traffic flow of a link is correlated to the traffic flow of its competitive links, the more the flow variation is in the link. Considering the network dependency, however, it is corroborated that the more the network dependency between a link and its competitive links, the more the flow variation in the link. This is also true for complementary links.

Towards a Metropolitan Fundamental Diagram using Travel Survey Data

Recently published:

a(t) vs. N(t) vs L(t) (2000 and 2010). 2010 shown as wider lines.
a(t) vs. N(t) vs L(t) (2000 and 2010). 2010 shown as wider lines.

Using travel diary data from 2000-2001 and 2010-12 this research examines fundamental traffic relationships at the metropolitan level. The results of this paper can help to explain the causes of some traffic phenomena. First, trip numbers on the network in the Minneapolis – St. Paul, Minnesota (Twin Cities) region show a bimodal diurnal pattern, with more trips in the afternoon. This relationship holds for full-time work and for non-work trips, but not part-time work trips. Second, network average speed by time of day can be explained by trip length and cumulative number of vehicles on the road. A clockwise hysteresis loop is found in the Metropolitan Fundamental Diagram in the morning period and a reverse process happens in the afternoon.

Key words: Macroscopic Fundamental Diagram, Network Fundamental Diagram, Traffic Data, Travel Surveys

New Book: The End of Traffic and the Future of Transport

We are pleased to announce the publication of our latest book The End of Traffic and the Future of Transport on Kindle Editions and at the iBookstore. The price is $4.99.

The End of Traffic and the Future of Transport, by David M. Levinson and Kevin J. Krizek
The End of Traffic and the Future of Transport, by David M. Levinson and Kevin J. Krizek

Table of Contents

  • Preface: The Lost Joy of Automobility
  • Climbing Mount Auto: The Rise of Cars in the 20th Century
  • Less Traffic is a Good Thing
  • What Killed America’s Traffic?
  • Pace of Change
  • Transitioning Toward Electric Vehicles
  • Autonomous Autos
  • MaaS Transport
  • Transit
  • Up and Out: The Future of Travel Demand and Where We Live
  • Adapting the Built Environment
  • Reduce, Reuse, Bicycle
  • Accelerating the End of Traffic via Pricing
  • Redeeming Transport
  • Post-script 1: What Happened to Traffic?
  • Post-script 2: Now extinct: the Traditional Transport Engineer

In this book we propose the welcome notion that traffic—as most people have come to know it—is ending and why. We depict a transport context in most communities where new opportunities are created by the collision of slow, medium, and fast moving technologies. We then unfold a framework to think more broadly about concepts of transport and accessibility. In this framework, transport systems are being augmented with a range of information technologies; it invokes fresh flows of goods and information. We discuss large scale trends that are revolutionizing the transport landscape: electrification, automation, the sharing economy, and big data. Based on all of this, the final chapters offer strategies to shape the future of infrastructure needs and priorities.

We aim for a quick read—and to encourage you and other readers to think outside your immediate realm. By the end of this book (today, if you so choose) you will appreciate the changing times in which you live. You will hopefully appreciate what is new about transport discussions and how definitions of accessibility are being reframed. You will be provided with new ways of thinking about the planning of transport infrastructure that coincide with this changing landscape. Even if transport is not your bailiwick, we like to think there is something interesting for you here. We aim to share new perspectives and reframe debates about the future of transport in cities.

Open Source Trip Generation

We have long known in the transportation planning community that the use of trip generation for local area review, and ITE’s procedure for estimating trip generation is broken in any number of ways. Shoup’s Truth in Transportation Planning is a classic critique of the problems.

While we could (and perhaps should) throw the whole kit and caboodle into recycling, in practice trip generation methods will be with us decades from now (even as traditional work, shopping and driving disappear). So there is a small academic movement to make the methods better. The most recent issue of JTLU 8(1) has a special section on Trip Generation, including several papers about how to adjust and improve ITE’s Trip Generation methods based on better data.

Part of the problem is that ITE is functionally a for-profit organization, and makes bank on selling the Trip Generation Manual and associated software (recognizing the fact that use of ITE Trip Generation rates is ensconced in law and regulation).

What has long been needed is an open source database of trip generation studies so that better fits to actual site conditions can be used in analysis. I recall in my youth some engineers in Montgomery County, Maryland trying to set something up, but this was well before the world wide web made that easy.

Fortunately that day is upon us. Mike Spack and company have set up, which is populated with open access trip generation studies (licensed under a Creative Commons license), and for which they hope to grow the data set. This is new, and I assume as it grows the data will get better and better, as will the methods for inputting and extracting data. Kudos to Mike, Nate, and others at Spack Consulting for getting this going. I look forward to seeing where this goes, as Big Data and new sensors make data collection increasingly ubiquitous.

The Road to Vision Zero Has Some Bumps In It | NY City Lens

Elena Boffetta writes in NY City Lens The Road to Vision Zero Has Some Bumps In It

Speed humps are proposed in Sunnyside. I note there are alternatives.

David Levinson, a professor in the Department of Civil Engineering at the University of Minnesota, said speed humps are not the most efficient way to slow down traffic, as drivers get used to them and tend to speed after passing one, or just avoid them by using alternate routes.

Levinson said speed humps are only one part of a measure called traffic calming, which is a change in the infrastructure and environment of the roads to slow down traffic and make the streets safer for bikers and pedestrians. He said there are other more effective forms of traffic calming.

“Other solutions would be putting trees on the side of the road, changing the pavement material, putting on-street parking,” Levinson said. “A very good one is to narrow the streets intersections. If the intersection is narrow the sidewalk is extended and there is a change in the environment, so cars need to go slower because they are driving through a narrower region.”

He said speed humps also create difficulties for fire trucks, garbage removal vehicles, and snowplows. He said one solution to lower speeds and fewer accidents in residential areas would be to follow the woonerf movement in use in the Netherlands, a system of “living streets” where pedestrians and cyclists have legal priority over motorists.

Are our roads really more congested | StarTribune

Tim Harlow, of the StarTribune’s The Drive column asks if “our roads are really more congested?” and interviewed me:


Of course, almost every time a report on traffic comes out, it makes the headlines. But how do we make heads or tails out of them when they seem to contradict one another?

“They are not necessarily contradictory,” said David Levinson, a professor of civil engineering at the University of Minnesota. “They all have different data, but they are measuring roughly the same thing.”

The differences can start with how the report makers define congestion. MnDOT defines congestion as traffic flowing at speeds less than or equal to 45 miles per hour. TomTom defines it as increased travel time when compared to free-flow conditions.

The data used to compile reports comes from different sources, too. MnDOT uses loop detectors embedded in the pavement while INRIX uses GPS data. Another difference is that the INRIX report looked at traffic volume and delays while MnDOT’s congestion report details the location and percentage of freeways experiencing daily congestion.

New business or housing developments can alter traffic flow in areas, making roads that were adequate suddenly become packed, creating the perception that congestion is getting worse.

“You might see more this year because the economy has picked up, but generally it has been flat and has been for a while,” Levinson said. “If it gets too bad, people will change their behavior. … There are limits on how bad the congestion can get.”

Minneapolis-St. Paul is No. 16 on the list of America’s worst traffic cities | Star-Tribune

I was interviewed by Mary Lynn Smith of the Star Tribune for the annual INRIX congestion report article:
Minneapolis-St. Paul is No. 16 on the list of America’s worst traffic cities

Depending on the methodology, rankings put the Twin Cities between the 13th- and 16th-largest U.S. metro area, said David Levinson, a civil engineering professor at the University of Minnesota. “The fact that we’re ranked 16th in congestion seems about right,” he said.

Levinson said demographic trends are helping to mitigate road congestion.

“Travel times are declining in the U.S.,” he said. “People are aging. Old people don’t travel as much, and young people don’t travel as much as what young people used to. Fewer kids own cars. The big picture is that that the total amount of travel peaked in the U.S. a few years ago and it’s been declining ever since. We have some ups and downs during any given year depending on the price of the gas and whether the economy is doing a little bit better or not. Certainly [congestion is] more than in 2009 during the depths of the recession.”

Levinson and others are quick to point out that Twin Cities drivers could be dealing with much worse.

In Los Angeles, home to the nation’s most-congested roads, drivers spent 64 hours sitting in traffic, an increase of five hours from the previous year, according to the INRIX study. In Honolulu, the nation’s second-worst city for traffic, drivers sat behind the wheel 60 extra hours last year, while in No. 3 San Francisco it was 56 hours.

And, Levinson points out, there’s more good news for the Twin Cities. The average speed of travel in the metro area is the fifth-highest in the country.

“You sit in traffic at a particular bottleneck, but then when you’re moving on the freeway, you’re driving at 55 mph,’’ he said. “And when you’re driving on arterials, you’re driving at 45 mph, and that’s better than most metro cities.”