This project is designed to provide a common workspace to post information about roadway capacity reductions.

The effects of roadway capacity reductions is a controversial issue in the world of traffic planning.  The effects on congestion of reducing roadway capacity has very large policy implications.  If reducing roadway capacity causes unstopable chaos, then our policy options are very limited.  However, if reducing roadway capacity does not cause traffic chaos, then as a society we have a large variety of policy options available to us. 

The seminal study on this issue is summarized Sally Cairns, Steven Akins and Phil Goodwin, Disappearing Traffic: The Story So Far, Centre for Transport Studies, University College London http://www.ucl.ac.uk/transport-studies/tsu/tpab9908.htm

The only problem with the Cairns study is that it is light on New York specific examples.  Here is a partial list of cases in NY where roadway capacity was reduced.  Please feel free to add to this list and to add details about each case.  Please help to document predictions about what would happen in these cases and then how accurate those predictions proved to be.

Examples of Roadway Capacity Reduction in NYC:

Closure of Washington Square Park to Traffic

Miller Highway Collapse (West Side Highway)

Closure of specific entrances to Central and Prospect Parks

Williamsburg Bridge, removal from sevice

Queens Blvd reengineering.

Long Island City, Queens, during a recent (1990's or 2000') construction project, Brian Ketchum knows something about this.

Clinton Street and Union Street water main project in Brooklyn around 2000-2002



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I've been looking for hard data that proves shrinkage, and I haven't found that much that I consider fully objective. Some papers, such as Cairns/Goodwin “Disappearing Traffic”, cite specific data, but the sources of the data are not included in the article. (I'm working on finding these sources, that will be far more convincing).  Until I have the source data in my hands, I don't think we can expect DOT to believe either the methodology or the facts as provided by a crazy liberal think-tank like ourselves.

As such, I wouldn't consider any reports produced by an anti-traffic group (such as this group at University College London, or TA, etc) as usable evidence in a case against the DOT, for obvious reasons of subjectivity. I am still searching for data that is empirically accurate with clear, objective methodology that describes the phenomenon of shrinkage. My hope is to find some articles in academic journals, such as the Journal of Transportation Research or the Journal of the American Society of Civil Engineering, about reduced capacity as it relates to reduced volumes.  I'm just starting to find some good stuff in the British Traffic Engineering and Control Magazine, but I'm waiting on the actual articles (only have titles for now). A lot of this data for catastrophic events in New York, like the West Side highway collapse volumes, can be requested; it's just going to take awhile.

 
On the other hand, where data for shrinkage is tough to come by, evidence for the concept of induced capacity, whereby new roadway space attracts more cars, is thoroughly described in journals and traffic studies and is far more available. My worry is that many articles that support our cause cite the data behind capacity induction and take the contrary, that a reduction in roadway spaces discourages cars, to be automatically true. This seems to be a flaw in logic (we cannot prove that if an “If A, then B” argument is true, that the contrary, “If not B, then not A” is also true). So...we have to prove the argument behind traffic shrinkage in its own right.

And to that end, I’m still searching the recesses of the internet and transportation statistics libraries for these numbers.  I bet that with most of the data we can find, the engineers at DOT will argue that the before and after traffic count methodology was not the same (which maybe be true). For example, even if we can find good traffic counts for after the West Side Highway collapse, it may be tough to find data collected after the event with the same methodology.

I've put below what I think are some relevant articles and studies. They don't prove the case of shrinkage per se, but they are interesting and might set us on the right path. Some are much more relevant to induced capacity rather than shrinkage (see notes):

Chinatown Access and Circulation Study

http://www.renewnyc.com/content/pdfs/Chinatown_Final_Report_2004-12-13.pdf

This report assesses how the closure of Park Row after 9/11 affected traffic patterns and volumes in the Lower Manhattan area. Report is prepared by Parsons Brinckheroff  (traffic engineer) for the LDMC. Most notable is a mention about how permit parking by government employees causes congestion in the area (though the report suggests building a private garage to accommodate public employees rather than removing parking spaces and encouraging transit). Also, the report attempts to qualitatively explain changes in traffic volumes (decreases) after the closures (ie “there is not enough data to quantitatively say where the cars traveling on broadway went, but we assume that the cars were diverted to other north-south corridors”). I’m a little troubled that an engineering firm is making assumptions in a final report, especially when the assumption is that traffic was diverted (no mention that it could have disappeared). Shrinkage doesn't enter their minds!

PARKING AND LIVABILITY IN DOWNTOWN SAN FRANCISCO
Parking policies to discourage congestion and improve the urban environment in the new, mixed-use downtown

http://www.spur.org/documents/pdf/050101_report_01.pdf

Fantastic report put out by the San Francisco Planning and Urban Research Association that details how parking policy can affect congestion. Report includes data on how employees respond to parking incentives (similar to government workers receiving parking permits in NYC, how the availability of parking at a destination is the second most important factor in determining mode of transport, after the actual cost of parking, and how reduction of incentives and parking capacity inevitably increases carpools and transit ridership.

ACCOUNTING FOR INDUCED TRAVEL IN EVALUATION OF URBAN HIGHWAY EXPANSION

http://ntl.bts.gov/lib/000/700/700/00780098.pdf

Article about induced capacity, with emphasis on traffic modeling techniques and methods to model potential induced capacity. This doesn't help the case for shrinkage, but does go into the methodology behind modeling.

2004 NYCDOT BRIDGE TRAFFIC VOLUMES

http://www.nyc.gov/html/dot/pdf/bridgetrafrpt04.pdf

NYCDOT collects bridge traffic volumes every year and has done so for quite some time. The older data is not available on the website, but is assumably obtainable. Hopefully comparisons pre- and post-Williamsburg bridge closure will be interesting and will not show a massive jump in traffic on the other East River crossings.

Traffic Technical Report, West Side Highway Project , New York Department of Transportation.

September, 1976. I will FOIA this. It could be very helpful.

Remove It and They Will Disappear (Surface Transportation and Policy Institute)

http://ntl.bts.gov/lib/1000/1100/1165/00778490.pdf

Report on shrinkage, cites evidence, and the Traffic Technical Report (listed above) is in the bibliography. Report is very good and proves our point, but the STPI is pretty liberal, so we can't exactly cite this as "fact" for DOT.

New Thinking for a New Transportation Age, put out by the Local Government Commission

http://www.lgc.org/freepub/PDF/Land_Use/focus/new_transportation_age.pdf

Has a section on how increased capacity induces traffic beyond the rate of population growth, and how conversely, many cities are closing bridges and roads and actually are seeing a decline in auto trips.

Victoria Traffic Policy Institute, Bibliography on Induced Traffic:

http://www.vtpi.org/induced_bib.htm

could be good sources, look into further.

Transportation Elasticities: How Prices and Other Factors Affect Travel Behavior (Victoria Traffic Policy Institute)

http://www.vtpi.org/elasticities.pdf

In depth examination of traffic elasticity, does not specifically deal with induced demand, reduced capacity. Discusses how several factors (price of commute, travel time, etc) affect transit decisions, essentially a case-study of opportunity cost with respect to transportation modes. Specifically refers to a study by UK Economists that claims that when travel time is reduced, demand increases, and vice versa. Could be a good argument for elasticity of traffic demand when capacity is decreased, conversely.

Generated Traffic and Induced Travel: Implications for Transport Planning (Victoria Traffic Policy Institute)

http://www.vtpi.org/gentraf.pdf

Interesting report on the elasticity of traffic, especially how cars respond to available road space: demand increases in pace with capacity. Also, article contains some specific references to traffic modeling techniques.

TravelSmart UK

http://www.eltis.org/study_sheet.phtml?study_id=630

Firm in the UK contacts people directly offering to make a custom transit plan for the household, with the idea that people would use alternative forms of transportation if they were educated or informed of options and routes. In areas with pilot programs, car use decreased between 6 and 9%. Not relevant to proving the case of shrinkage, but it demonstrates that demand is elastic based on education re: various travel options.

Relevant studies, reports, articles from aboutcss.org

Disapearing Traffic: The Story So Far -- Full, updated report with executive summary
http://www.aboutcss.org/content/reading/disappearing-traffic/

Traffic Impact of Highway Capacity Reductions: Assessment of the Evidence 
http://www.cts.ucl.ac.uk/tsu/tpab9828.htm
By the same authoThis study covered 60 instances in the US and abroad of where roadway capacity had been reduced. It found that "Prolonged, long-term gridlock is simply not reported, although there can be short-term disruption, and some increase in problems on particular local roads. In many cases, there were actually significant reductions in the total amount of traffic on the networks studied. On average, 14-25% of the traffic that used to use the affected route, could not be found on the neighbouring streets.

Road Diets: Mobility and Access Improve  -  Dan Burden's description of road diets and their benefits
http://www.aboutcss.org/content/reading/road-diets-4/
Comprehenisive report on Road Diets with many case studies from Walkable Communities:
http://www.aboutcss.org/content/reading/road-diets-3/resources/road-diets-fixing/

Evaluation of Lane Reduction "Road Diet" Measures and Their Effects on Crashes and Injuries
http://www.aboutcss.org/content/reading/road_diet_effects/
Study explores the benefits of fewer lanes for vehicles-vehicle acccidents and for vehicle-pedestrain accidents.

Road Diets: Four-Lane to Two-Lane Conversions
http://www.aboutcss.org/content/reading/road-diets-2/
Examples of where roads have been reduced to 2 lanes from 4, and capacity has not been impacted.

Clinton Square Case Study, Syracuse, NY -- Institute for Traffic Enginneers
http://www.pps.org/trans/info/clinton_square_ite
Clossing a major road to throughthe center for Syracuse to creata  sqaure was found not to have negaive effects on capactiy o or congestion of sorounding streets.  PPS helped develop the vision for the square that closed the road.  This miht be somewhat relevent to Central Park as well.

Relevant quotes from www.aboutcss.org:

"If all we're doing is addressing shorter times for travel greater distances, then we're not solving our long term mobility. Shorter trips and more access is the way to address mobility problems." - Dan Burden

"Increased capacity was supposed to ease traffic. Instead, it keeps attracting more cars."  - Project for Public Spaces (PPS)