Elements of Access: One-Way Streets

by Wes Marshall.



Why do traffic engineers seem to like one-way streets so much? The AASHTO Green Book points out a handful of efficiency advantages [1]. By removing the delay caused by left-turning cars, we increase traffic capacity and speed. Fewer intersection conflicts means more efficient signal timings and, in theory, fewer and less severe crashes (e.g. by eliminating head-on crashes). Medians are no longer necessary, so you can often fit in an extra lane of through traffic, which further increases capacity and speed. More mobility with better safety? What’s not to love?

Beyond the abundant advantages, AASHTO lists a few disadvantages as well. There is the potential for increased travel distances in cases when you have to travel almost around a whole block to reach your destination. When all lanes begin to back up at traffic lights, emergency vehicles may be blocked. Lastly, one-way streets may confuse visitors.

Given AASHTO’s list of pluses and relatively few minuses, it makes sense why so many of our streets send traffic in just one-direction. Then again, it’s not hard to argue that what AASHTO deems an “advantage” might be the opposite. If I lived or worked on a one-way street, I’d be pretty hard-pressed to believe that more cars moving at higher speeds is necessarily a good thing.

Appleyard’s early studies found many residential livability advantages on the two-way streets, but the one-way street he investigated had far more traffic than the two-way comparisons [2]. Denver converted a handful of one-way pairs to two-way operation in the early 1990s and found that residents preferred the change [3]. A recent case study out of Louisville looked at a handful of one-way to two-way conversions and found significant increases in pedestrian traffic, property values, and business revenue [4]. These benefits were accompanied by a significant decrease in crime. Other cities such as Charleston, SC and Lubbock, TX also found success in terms of two-way streets helping downtown revitalization [5, 6].

Such livability benefits are all well and good, but are they worth the increased road safety risks that AASHTO made clear? The research is beginning to suggest that the safety answer isn’t clear cut. Lubbock found no significant change in terms of traffic volumes or safety [6]. Another study from Jerusalem also found no difference in road safety [7]. Despite similar traffic levels on the Louisville conversions, crash rates dropped with the two-way streets [4]. Moreover, child pedestrian injury rates on one-way streets have been found to be more than double the rates on two-way streets [8].

More research is needed on the safety outcomes. However, it is also interesting to ask why the safety benefits of one-way streets be overblown. First, there are likely to be differences in driver behavior, most notably with faster speeds on one-ways. It is pretty easy to understand see why slower traffic – despite the noted increase in conflict points – might help reduce crash severity. Another ITE guide even says the following regarding the safety of one-way streets: “one-way pairs with good signal progression and high travel speeds seemed to elicit red-light running behavior” [9].

The image below comes from the ITE Traffic Engineering Handbook. It makes the case for better safety on one-ways by depicting the number of conflict point at an intersection for a two-way street as 32 and for a one-way street as only 5. This is a stark difference that could theoretically result in better safety. Beyond the fact that conflict points are not often well correlated with actual safety outcomes [10], the bigger issue is that they are comparing apples and oranges. This diagram compares an intersection where all four-legs have two-lanes to an intersection where all four-legs have one-lane. In reality, the one-way streets would have at least two lanes, if not three as in the image from Denver above or in cases where the median is removed. One-way streets with multiple lanes is a fairer comparison that would substantially increase the number of potential conflict points and deem the comparison in the image below as meaningless.

Moreover with regard to conflicts, AASHTO even suggests converting from two-way to one-way operation in situations where an urban street has too many pedestrian-vehicle conflicts [1]. The reduction in pedestrian-vehicle conflicts is supposedly derived from a simpler set of intersection movements. The real reason for the reduction of pedestrian-vehicle conflicts might be even simpler: fewer pedestrians wanting to cross the street in the first place.

So after all that, the only definitive advantage left for one-way streets is increased traffic capacity. However, this point is also up for debate. Taking into account the decreased accessibility of circuitous routes, one paper found that drivers make significantly more turning movements and travel greater distances given the same origins and destinations in a network dominated by one-way traffic patterns [11]. Another more recent paper suggests that a network of two-way streets actually has a greater trip-serving capacity – particularly for trips less than 5 miles – as compared to a network of one-way streets [12]. When also prohibiting left-turns in the two-way network, this capacity advantage of the two-way network included longer trips as well.


Not only do one-way streets hinder accessibility and livability, but the traffic engineering benefits don’t necessarily seem to hold. While one-way streets are still needed when relatively narrow cross-sections prevent two-way traffic, in most other urban contexts, it is hard to imagine why so many cities continue to preserve one-way streets. Some cities are changing their ways. The before-and-after images at the top are from Larimer Street in Denver where a mile-long stretch was recently converted from a one-way into a two-way. Instead of three high-speed lanes heading toward downtown, there is now one lane in each direction with accompanying bike lanes. With noticeably slower traffic and more active transportation use along this corridor, it makes sense why there so many new businesses seem to be popping up, especially when compared to the parallel streets that remain focused on one-way traffic. It might be time for cities to find a new direction – and more research is needed – but it seems like this this new direction will run both ways.



  1. AASHTO, A Policy on Geometric Design of Highways and Streets. 2011, Washington D.C.: American Association of State Highway and Transportation Officials.
  2. Appleyard, D. and M. Lintell, The Environmental Quality of City Streets: The Residents’ Viewpoint. Journal of the American Institute of Planners, 1972. 38(2): p. 84-101.
  3. Robert Dorroh, I. and R. Kochevar, One-Way Conversions for Calming Denver’s Streets, in ITE International Conference. 1996, Institute of Transportation Engineers: Las Vegas, NV.
  4. Riggs, W. and J. Gilderbloom, Two-Way Street Conversion: Evidence of Increased Livability in Louisville. Journal of Planning Education & Research, forthcoming.
  5. Baco, M.E., One-Way to Two-Way Street Conversions as a Preservation and Downtown Revitalization Tool: The Case Study of Upper King Street, Charleston, South Carolina, in Historic Preservation. 2009, Clemson University and College of Charleston: Clemson, SC.
  6. Hart, J., Converting Back to Two-Way Streets in Downtown Lubbock. ITE Journal, 1998. August.
  7. Hocherman, I., A.S. Hakkert, and J. Bar-Ziv, Safety of One-Way Urban Street. Transportation Research Record, 1990. 1270: p. 22-27.
  8. Wazana, A., et al., Are Child Pedestrians at Increased Risk of Injury on One-Way Compared to Two-Way Streets? Canadian Journal of Public Health, 2000. 91(3): p. 201-206.
  9. ITE, Toolbox on Intersection Safety and Design. 2004, Washington, D.C.: Institute of Transportation Engineers.
  10. Jacobs, A.B., E. MacDonald, and Y. Rofe, The Boulevard Book: History, Evolution, Design of Multiway Boulevards. 2003, Cambridge, MA: The MIT Press.
  11. Walker, G.W., W.M. Kulash, and B.T. McHugh, Downtown Streets: Are We Strangling Ourselves on One-Way Networks? Transportation Research Circular, 2000(501).
  12. Gayah, V.V. and C.F. Daganzo, Analytical Capacity Comparison of One-Way and Two-Way Signalized Street Networks. Transportation Research Record, 2012. 2301: p. 76-85.
  13. ITE, Traffic Engineering Handbook. 5th ed. 1999, Washington, DC: Intstitute of Transportation Engineers.

Elements of Access: Why the home addresses of your friends are causing global warming

by Kay Axhausen

In the industrialized world leisure travel makes up about 40% of all trips and 40% of all distance travelled: the largest and fastest growing segment of the travel market in these societies. Leisure is a catch-all category in standard travel (diary) surveys: not work, not education, not shopping, not personal business, not bringing or dropping somebody off. It covers going many different activities: from window-shopping to meeting friends for a weekend hike.

Some of these leisure activities are regular, such as attendance at church or going to the gym, but others are irregular or unique: that visit to a friend last seen ten years ago, or going on the Hajj. These activities don’t have the same constraints as work or school, to which we are committed through a contract or a legal requirement. Some are spontaneous, but other expressed deeply held commitments, such as a pilgrimage or the gym. While we often think of them as “free”, their social nature makes them binding for us: 80-90% of these activities involve other persons: family, friends, the three other golfers, other players of team, the other 9 worshippers waiting for you to make the quorum in the synagogue; never mind the dog expecting her walk.

This overwhelming social nature of leisure implies that the activities are also joint decisions as one has to account for the efforts of the others, when one fixes dates and locations.  In some cases the choices have become so habitual, that the organizer does not think anymore, say for club, civic or religious events, but for most others the negotiation is a (large) part of the preparations; which can be seen in the large amount of SMS, email, telephone traffic involved beforehand.

As the effort involved in participating face-to-face in an event, meeting, party, get-together involves at minimum the travel to get to its place to spatial distribution of one friends come crucial. The wider our circles are the more travel, and associated with greenhouse gases will we produce. Yes, the higher the effort, the less likely that we will meet certain persons (See <Point>), but there will be a certain minimum frequency to honor our links: attendance at the wedding of a cousin, being at the funeral of your friend’s wife, the annual joint hike.

While sociologists have long studied the (social) structure of social networks, they gave little and generally cursory attention to the spatial distribution of the networks. Recent work by joint teams of transport planners and social scientist has shed light in the distances involved (See figure below).

Great Circle Distance
Frequency vs. Great Circle Distance

In this typical example from a Swiss study the bulk of social contacts lives within a 30 min car ride, but there is a substantial share living much further away, including some overseas. This distribution should add more long distance contacts as travel and communication becomes cheaper with low-cost flying and zero marginal cost voice-over-IP telephony. So, indeed the home addresses of our friends and our wish to meet them is one driver in greenhouse gas production.

Journal of Transport and Land Use Vol 8, No 1 (2015)

We are pleased to announce the release of the Journal of Transport and Land Use Vol 8, No 1 (2015), with two Special Sections, one on Trip Generation, edited by Susan Handy,  the other on Paratransit, edited by David King

Table of Contents

Special Section: Trip Generation

Susan L Handy
Kelly J. Clifton, Kristina M. Currans, Christopher D. Muhs
Adam Millard-Ball
Miguel Jaller, Xiaokun (Cara) Wang, Jose Holguin-Veras
Robert James Schneider, Kevan Shafizadeh, Susan L Handy
Kristina M Currans, Kelly J. Clifton

Special Section: Paratransit

David King
Pablo Salazar Ferro, Roger Behrens
Andreas Neumann, Daniel Röder, Johan W. Joubert
Nicholas J. Klein
Jonas De Vos

The Purpose of Peer Review

As a young teacher, I taught my first senior technical elective course without giving a final exam. I wanted the students to learn, and there were enough assignments. I got feedback from students that without a final, they wouldn’t study, or even read the text. So I give a final now. Not because I want to grade finals, or even use it as evaluation, but to incentivize students to actually read the readings.

This leads me to the hypothesis that the primary purpose of academic Peer Review is not to review papers and give feedback to authors. It is instead to induce authors to submit work of high quality because they believe someone will read it. In practice, the quality will be high enough  that the papers will have a chance of passing peer review.

In the absence of peer review, authors could submit any old junk and it would be published. But with review, authors can only submit higher quality pieces with the hope of getting published in a good journal.

Of course peer review does give feedback, this is the developmental component, as well as evaluation (accept, decline) which aids editors in deciding what to accept for publication. I generally dislike the developmental component. It is used to avoid a clear decision of accept or decline, forcing nearly everything into multiple rounds of review, and slowing progress. Further, knowing that almost nothing gets through on the first round, authors don’t submit their best work. I have heard famous faculty say they write papers to 80 or 90% completion, knowing the reviewers will have specific comments that cannot be foreseen, which will finalize the paper. Thus reviewers get incomplete work.

We should restore peer review to its purpose as evaluative.

No paper will ever be perfect, but with the current guarantee that nothing (ok, maybe 1 or 2%) is accepted in round 1, it is pointless to even try. As an editor I am trying to increase those odds, but reviewers do not help in this regard.

Søüthstår: What If the Southwestern Suburbs Were Served by Commuter Rail?

On Monday I talked about Shørtstår, transforming a section of the Northstar commuter rail line into actually useful transit. As with all transit right-of-way problems, “the railroads would never agree” was raised. And that is certainly true if all you do is politely ask.  However states are actually bigger and more powerful than railroads, though it may not seem so.  They have powers of taxation and eminent domain. They have a monopoly of force. They occasionally even have the will of the people. Sometimes states even own the land under the railroad, as is the case with a section of the Twin Cities and Western (TC&W), which runs a massive two trains per day in the critical Kenwood region.

We have recently learned (to no one’s surprise, I hope) that the costs have risen on the Southwest LRT.

Well, there is nothing to say you couldn’t run some passenger trains in that corridor today. It wouldn’t be LRT, and it wouldn’t always be double-tracked unless it were upgraded, and it wouldn’t have a 10 minute headway, but why not test service  in the corridor along the existing RR right-of-way and tracks? If it is successful, it can be expanded in frequency, the line can be double tracked in part or in full, more train-sets acquired, and permanent stations built. It wouldn’t require a tunnel in the Kenilworth Corridor or taking any homes or businesses. The TC&W could even be acquired for a hundred million dollars or so  if they weren’t cooperative.

Southstar - A theoretical, conceptual, non-official line on a map for a commuter rail serving some of the Southwestern suburbs of the Twin Cities
Søüthstår – A theoretical, conceptual, non-official line on a map for a commuter rail serving some of the Southwestern suburbs of the Twin Cities

So why call this Søüthstår? If you look at the Map, you could through-run the Northstar (or Shørtstår) trains on this line to the south and Southwestar doesn’t have the same ring.  Why the accents? That’s just to get the attention of the Governor.

Even if it isn’t successful, we just saved most of $2 Billion.

The current political class is not very good at thinking incrementally, but if something big is good, often something smaller that does similar things at a lower intensity is also good, and it might be more good per dollar spent. (Which thereby lets you do other things).

You ought not build half a lane, or a single rail (Monorail?), but there are increments of rail service (such as trains to Hopkins rather than Eden Prairie) that can be completed even if the whole cannot be.

I personally am skeptical of the whole thing, express buses should serve these destinations well, but if you are committed to trains, why wait until 2025? Just run some trains – demonstrate your point. Stop abiding by the tyranny of false constraints. Let’s turn Target Field Station into the Grand Central it was supposed to be.

What is the capacity of the Green Line?

While reading an excellent article by Yonah Freemark, Why should Chicago focus growth near transit?, I thought the Twin Cities should do the same thing. Taking advantage of existing capacity is far more cost effective than building new capacity (and yes, this applies to all modes). But what is the existing capacity of the Green Line? Well, that depends on assumptions and human behavior. In the table below I work through some scenarios based on assumptions.

First, how many hours per day is the Green Line operating? Second, what is the frequency within that time period? Third, how many cars per train are there? Fourth, what is the capacity per car (they are rated at 230, but this includes standees)? Fifth, how long is the line? Sixth, how long (how many stations) is the average trip? Seventh, how many directions are you considering?

This measures capacity in terms of daily boardings. Daily miles traveled is another measure, and is independent of the length of trips.

To calculate this we use the following equation:

Capacity = (Hours of Operation)*(Trains/Hour)*(Cars/Train)*(Capacity/Car)*(Stations – 1) * (Trip Length) * (Directions Operating).

At any rate, the attached table shows some surprisingly high numbers, up to 7 million (under the admittedly silly unconstrained scenario (A) where people only ride the train for 1 stop before alighting, trains run for 24 hours a day, and people are standing at near crush capacity), with more plausible numbers in the 255,000 territory, assuming everyone gets a seat, but you can run at 5 minute headways (C). Here we are limited by capacity in one section (downtown Minneapolis), which does run at 5 minute headways, but splits the capacity between the Green and Blue lines.

The main point is that there is a lot of capacity on the Green Line yet to go, even if you only run 18 hours a day, and you expect everyone to have a seat, and run at today’s 10 minute headways (which is all today’s fleet can support, to increase headway we either need to increase speed greatly or add vehicles), and assume the average trip is 7 stations (Aaron Isaacs informs me it is 3.5 miles, which at 1/2 mile spacing is about 7 stations) (83,314 – scenario D). At the other end of the spectrum, if everyone expected a seat and was riding from Union Depot to Target Field, the capacity would only be 32,400 with today’s frequencies.

Thus, east–west transportation capacity is not the constraint in development along the Green Line corridor. (One could similarly demonstrate the under-utilization in the north-south direction on buses, and in all directions on roads).

Certainly load balancing is an issue, much of the capacity is “off-peak”, but that is what pricing is for. Higher loads would increase wear and tear on the cars, and add costs, but hopefully the added revenues would more than compensate.

Compare with current ridership of about 37,835/day (Sept. 2014).

Given there is also a lot of developable land in this corridor, why are new corridors being subsidized for development? [I do actually know the answer to this, it was a rhetorical question].



Cross-posted at streets.mn.

Shørtstår: What if the Northstar were a local

Imagine you were in a region that was growing and had a transportation problem.

Shortstar Routing and Stations
Shørtstår Routing and Stations

Imagine you had a grade separated rail line into the heart of the city, connecting with other rail lines.

Imagine this line already had two stations constructed.

Imagine this line passed through some high density neighborhoods without stopping.

Imagine you already ran some service on this line.

Wouldn’t you look at this as an opportunity?

For a variety of reasons, the Northstar has not been the most successful transit line in the Twin Cities region.

While it runs through the streetcar suburbs and transit-compatible neighborhoods of Northeast Minneapolis, it doesn’t actually stop there. This diminishes the number of riders it might carry so that it might convey about 1000 suburbanites into downtown about 5 minutes faster. Both local residents going southbound in the morning, and suburbanites who might want to stop short of downtown get short-shifted by this configuration. There have been efforts in other cites to convert commuter trains into more frequent, all-day, urban service. (e.g. London, Toronto – add more in the comments). With only one line, that opportunity has not yet befallen the Twin Cities.

Imagine instead that instead of no stops, an abbreviated version of the Northstar line (let’s call it the Shørtstår Line, though we can give it a color like “Silver” or “Noir”) ran frequently during the day on the same corridor but on a reduced route between say Fridley and Target Field, and had stops at

  • Fridley
  • Lowry and 7th St NE,
  • Broadway and Central, and
  • University Avenue and 3rd Ave NE
  • Target Field Station

Like any good transit service, this would be a 10 minute headway service, served by several Diesel Multiple Unit trains. From aerial photos it appears the right-of-way should be sufficient along most of the route to provide two passenger-only tracks (since that is what our region insists upon). In any case, it could share tracks with freight most of the day, as the tracks do not appear to be congested in this region, if there were some forethought about scheduling. (Bigger cities do more with less).

What kind of ridership might this line get? I don’t know – but as lines go, this looks at least as plausible as many much more expensive routes that are being considered. For the cost of a few temporary stations, renting some trains, and some negotiating with the railroads for running rights for a one year trial, the region would get a good idea of how well this might work before expensive rolling stock were purchased and new tracks laid. If it worked well (in terms of cost-effectiveness, compared with other existing and proposed lines), more permanent infrastructure could be built. If not, the trains’ leases could expire and they could go elsewhere.

Cross-posted at streets.mn

U researcher rates MN’s travel accessibility

Brian Edwards at the MnDaily writes: “U researcher rates MN’s travel accessibility
 A University of Minnesota researcher is using travel data to rank the best areas in the state to live based on access to vital destinations.
The University’s Accessibility Observatory is evaluating transportation destinations, such as jobs, schools and hospitals in the state in order to measure accessibility.
The data could shape how entities like the Minnesota Department of Transportation plan future transit projects.
Andrew Owen, lead researcher and director of the observatory in the University’s Department of Civil, Environmental and Geo-Engineering, said the research identifies where jobs are concentrated.
“Focusing on accessibility gives a way to look at how well we are achieving the goals of transportation systems,” he said.
The program uses bus, rail, car and pedestrian travel times combined with census data to measure the number of jobs that can be reached within 30 minutes of a person’s home, Owen said. The data can be adjusted to give information about any type of destination from anywhere in the state.
David Levinson, a professor in the Department of Civil, Environmental and Geo-Engineering, said this information can also explain why people choose a certain mode of transportation.
“In places with higher transit accessibility, people are more likely to use [public] transit,” he said.
Levinson said the research also focuses on how frequently public transportation is available at a certain location.
“Transit accessibility varies by time of day,” Levinson said. “If the bus just left and won’t be back for another 30 minutes, you can’t reach very many places.”

ITSO TransTalk Seminar: April 24: What happens downstream of a bottleneck does not always stay downstream.

Benjamin Coifman will be giving an ITSO TransTalk seminar on Friday April 24 in the Civil Engineering Building (500 Pillsbury Drive) Room 205 at 12:15. Food will be provided.


What happens downstream of a bottleneck does not always stay downstream.


In modern cities freeway traffic congestion degrades the movement of most persons and goods. The congestion is due to a small number of bottlenecks and just as a chain is only as strong as the weakest link, freeway flow along a corridor is restricted by the tightest bottleneck. Conventionally bottlenecks are modeled as a point along the roadway with queuing upstream and free flow downstream. Downstream of the bottleneck all signals are presumed to flow downstream with the traffic while within the queue many signals propagate upstream (e.g., stop and go traffic). This talk presents two detailed examples where this conventional wisdom fails to capture the microscopic details of the actual traffic dynamics where disturbances actually propagate upstream through the bottleneck from the supposedly free flow conditions downstream. Unfortunately the small misunderstandings have lead to large errors in the conclusions reached by many researchers. The first example presents empirical evidence of subtle flow limiting and speed reducing phenomena more than a mile downstream of a lane drop bottleneck. These phenomena reduce the maximum throughput measured at the lane drop bottleneck below actual capacity, so in this case conventional measures underestimate capacity.

The second example presents a simulation-based study of an on-ramp bottleneck. In this case the modeling incorporates driver relaxation whereby drivers will tolerate a truncated headway for a little while after an entrance but slowly relax back to their preferred speed-spacing relationship. The results show that flow downstream of the on-ramp bottleneck is supersaturated, so in this case conventional measures overestimate capacity. Thus, an empirical study or traffic responsive ramp meter could easily mistake the supersaturated flows to be the bottleneck’s capacity flow, when in fact these supersaturated flows are unsustainable and simply represent system loading during the earliest portion of bottleneck activation. Instead of flow dropping “from capacity”, we see flow drop “to capacity” from supersaturation.


Benjamin Coifman grew up in Minneapolis, graduated Suma Cum Laude from the University of Minnesota, earned a MS and PhD in Civil Engineering and a MEng in Electrical Engineering and Computer Science at the University of California, Berkeley. Currently holds a joint appointment in Civil Engineering and Electrical and Computer Engineering at the Ohio State University. Research emphasis on: Traffic Flow Theory, Traffic Monitoring, and Intelligent Transportation Systems.