Oops. I have been asked to open the plenary on “Urban mobility: Achieving social efficiency” at next week's Smart Cities conference in Barcelona (full details on which available here , and one of the central themes of the talk is the high importance of taking a strategic approach to slowing down and smoothing traffic in cities. As part of my due diligence I decided to check out the Zone 30 and Twenty is Plenty entries in Wikipedia. Where I found to my disappointment: (a) that there was no entry on Zone 30 in English (and if in French, German, Italian and Dutch, not (yet) in Portuguese, Spanish, etc.) and (b) nothing at all on the important Twenty Is Plenty program out of the UK.
After all Wikipedia is, like it or not, the most consulted first stop reference point by many, so I thought it might be an idea to draw this embarrassing void to the attention of my well-placed international colleagues via the World Streets Facebook page to see who might be able to help. And the first to step forward was Tom Bertulis who late last night sat down and worked up an excellent first stage entry on Zone 30 in English which follows here. You are cordially invited to comment and make suggestions for improvements of the reference, which you can either (a) add directly on the WP site or (b) here by using the Comments button above. But in this first stage I vote that we treat Tom as our project editor for this exercise and address him on this directly via email@example.com. And Tom? We all thank you.
The 30 kph Zone, or 20 mph Zone, is a form of urban intervention where an area is designated with a 30 kilometer per hour (kph) limit, or 20 mile per hour (mph) limit. Note that 30 kph equals 19 mph, but 20 mph is more commonly used. The 20 mile per hour zone can be through signing and/or through physical traffic calming measures. 
Some research points out that simply signing streets has little effect in slowing speeds. Moreover, more comprehensive measures such as implementing vertical and horizontal traffic calming measures and new roadway surfacing such as the brick surfacing so common in Dutch 30 kph zones can be cost-prohibitive for communities with limited resources.
Research has shown that reducing driver speeds in built-up areas reduce injuries for all road users, including motorists, bicyclists, and pedestrians. Aside from safety benefits, bicyclists and pedestrians tend to be more comfortable and more prevalent in areas with 20 mph speeds. Other studies have revealed that lower speeds reduce community severance caused by high speed roads. Research has shown that there is more neighborhood interaction and community cohesion when speeds are reduced to 20 mph. 
Safety effects of lower speeds
The link between vehicle speed and pedestrian crash severity has been established by research studies, with crash severity increasing as a function of motor vehicle speeds. If a vehicle hits a pedestrian while traveling 15 mph, most pedestrians will survive a crash, often sustaining only minor injuries. Minor increases in impact speed have been shown to have a profound effect on crash severity. At 25 mph, almost all crashes result in severe injuries and roughly half are fatal; and at 40 mph, fully 90% of crashes are fatal. The dramatic differences in fatality rates are a key part of the theory behind 20 mph zones. 
The driving philosophy behind a 20 mph zone is that it considers the streets in the zone to be a public space that seeks to strike a balance between the realities of an urban area bustling with pedestrian activity and the circulatory function of the roadways. It is considered to be a space for people who live, work, play and study in the area, not for people who cross the zone to get somewhere else. The theory is to reduce rat running while improving the safety and quality of life in the area.
The objectives of the implementation of a 20 mph Zone are:
- Provide safe street crossings
- Improve the quality of life
- Increase levels of walking and cycling
- Reduce obesity through increased active living
- Reduce rat running and cut through traffic
- Reducing traffic volumes and speeds motor
- To reduce road crash rates, injuries and fatalities to all road users
- Reduce greenhouse gas emissions, air pollution and noise pollution
- Foment an area where pedestrians, cyclists and motorists coexist safely and comfortably
- Develop public space that is open and safe for everyone, including people with disabilities
- Increase the space available for walking, biking, and people on the street to eat, play and enjoy life
- Provide a safe area for children in school zones
- Increase real estate values of local homes and businesses
- Increase the economic vitality of the area
- Strengthen the sense of community 
In European countries 30 kph zones have been used widely. On September 1, 1992, the city of Graz, Austria, became the first European city to implement a city-wide 30 kph limit on all roads except it's largest.
Significant 30 kph zones are ubiquitous across the Netherlands and are gaining popularity in the UK. In Switzerland 30 kph zones have been allowed by law since 1989 and they were first established in Zürich in 1991. 
To date, the popularity of 20 mph zones in the United Kingdom has not yet caught on in the United States. By one estimate, some 3 million people live in areas with 20 mph speed limits in the UK.
Outside of Europe, the traffic calmed zones are catching on but slowly. Mexico City is considering a proposal for a one square kilometer “Zona 30” area in a central part of the city known as La Colonia Roma.
In the US, 20 mph speed limits exist along linear routes, but are slow to catch on for area-wide implementation. New York City is leading the way with neighborhood-scale 20 mph zones and is currently implementing 60 miles of streets per year for conversion to 20 mph zones. 
Ten US states already allow 15 mph or 20 mph speed limits for linear routes, as follows:
Alaska stipulates 15 mph speed limits in alleys and 20 mph limits in business districts.
In Delaware school zones have 20 mph speed limits.
Florida has school zones which usually have 10 mph to 20 mph limits. Most use signing and flashing yellow lights during school times, but there is debate surrounding the efficacy of these measures.
Massachusetts has set their default speed limit at 15 mph in the vicinity of a mobile vendor with flashing yellow lights (such as an Ice Cream Truck) and at 20 mph in a school zone when children are present.
In North Carolina, the Central Business Districts (CBDs) have a statutory speed limit of 20 mph unless otherwise posted. They use "Reduce Speed Ahead" signage instead of the more common "Reduced Speed Ahead" signage.
In Oregon, rather than having a “when children are present” speed limit, they have a 20 mph speed limit with a time-of-day system, usually school days, 7 a.m. to 5 p.m. The speed limit is for school roads with posted speeds of 30 mph or below.
Pennsylvania generally uses 15 mph speed limits for school zones during arrival and departure times.
In Rhode Island the default speed limit is 20 mph within 300 feet of a school, which starts to emulate a 20 mph zone but is not an area-wide speed limit.
In West Virginia school zones have a statutory speed limit of 15 mph, except for roads with a speed limit of 55 mph or higher, which have an advisory speed of 35 mph in school zones when children are present. A school zone includes 200 feet adjacent to the school (or school road) in both directions.
Wisconsin has a default speed limit of 15 mph in school zones, near parks with children, and in alleyways.
- ^ Elizabeth Press (2010-08-30). "No Need for Speed: 20′s Plenty for Us". Streetfilms. Retrieved 2011-11-27.
- ^ Joshua Hart. "Driven To Excess: A Study of Motor Vehicle Impacts on Three Streets in Bristol UK". www.walk21.com. Retrieved 2011-11-27.
- ^"References - Pedestrian and Bicyclist Intersection Safety Indices, November 2006 - FHWA-HRT-06-125". Fhwa.dot.gov. Retrieved 2011-11-27.
- ^"20's Plenty for Us". 20splentyforus.org.uk. Retrieved 2011-11-27.
- ^"Tempo 30" (in German). www.stadt-zuerich.ch. Retrieved 2011-11-27.
- ^ Fried, Ben (2010-08-16). "NYCDOT Releases Landmark Ped Safety Study, Will Pilot 20 MPH Zones | Streetsblog New York City". Streetsblog.org. Retrieved 2011-11-27.
- ^"Chapter 346 – Rules of the Road". Wisconsin State Legislature. Retrieved 2011-11-27.
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About the author:
Tom Bertulis is a civil engineer with over 10 years of experience in the private, public, and non-profit sectors. He has led path projects, signal synchronization projects, traffic analysis studies, and cycle network implementations. His focus on creating "people scaled environments" started when living in Munich in 1992 and continued as an advisor to the Mayor of Seattle on bicycle issues from 2000 to 2003. Tom has been involved in non-motorized transport projects in several cities across Latin America. Tom worked on sustainable transport projects for ITDP Mexico, which he joined after working 3 years as the Engineering Manager at Cycling Scotland in Glasgow. . He holds a bachelor’s degree in Civil Engineering from Santa Clara University, is a Registered Professional Engineer (PE) in the State of Washington, USA, and is currently pursuing a graduate degree in Civil Engineering (with a focus on Sustainable Transport) at Northeastern University in Boston, MA. . He can be reached at firstname.lastname@example.org.
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