Speed Reductions

Strategies that Reduce Traffic Speeds

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TDM Encyclopedia

Victoria Transport Policy Institute

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About This Encyclopedia

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Updated March 8, 2007

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This chapter discusses various strategies for reducing traffic speeds, their effects on total vehicle travel and other impacts.

Description

Various strategies can help reduce traffic speeds:

· Traffic Calming involves roadway designs that reduce traffic speeds.

· Speed limits can be reduced.

· Speed enforcement can be improved, using conventional or newer techniques described below.

· New and existing streets can be designed to be narrower (DKS Associates).

· Signage can be improved, including signs that display the speed of vehicles as they drive by.

· Various strategies can be used to reduce traffic speeds on arterials, highways and rural roadways (DEA, 1999; CORDIS, 1999; Kamyb, et al, 2003).

· Driver education and public service announcements can be used to discourage speeding.

· Traffic signals synchronization can be optimized for lower traffic speeds. Signal progression on urban arterials can be set so that vehicles that maintain legal speeds avoid stopping.

· Vehicles can incorporate a system that automatically alerts drivers when they exceed speed limits, or prevents speeding altogether.

Drivers tend to maintain a speed that feels comfortable, based on the design (lane width, visibility, clearance) and use (traffic volumes, turning activity, pedestrian activity) of each stretch of roadway. As a result, simply reducing posted speed limits may do little to reduce actual traffic speeds. Effective speed reduction generally requires changing roadway design, or significantly increasing enforcement.

Below are common traffic law enforcement techniques (IIHS, 2000; Coulter Transportation Consulting, 2004):

· Radar. Police radar transmits a microwave signal at a known frequency and then receives the signal reflected back from the object. The signal frequency returns to the radar unit in proportion to the speed of the moving vehicle, and the vehicle's speed is displayed. Radar signals also can be used to trigger roadside warning signs that display vehicle speeds to drivers.

· Laser. A narrow band of light is transmitted to a targeted vehicle and returned by it, and the speed of the vehicle is displayed for the officer. These devices are similar in size and weight to police radar. LIDAR (Light Distance and Ranging) devices send out and retrieve a rapid series of light pulses to and from a target vehicle, using a time/distance calculation to measure speed based on the change in consecutive measurements. Laser devices have been used in the United States for speed enforcement since 1990.

· Speed Cameras. Radar signals can be used to trigger cameras that photograph speeding vehicles as they pass a specified point. These devices use a low-powered doppler radar speed sensor to detect speeding vehicles and trigger a motor-driven camera and flash unit to photograph vehicles traveling faster than a set speed. The date, time, and speed are recorded along with a photo.

· VASCAR. A vehicle average speed calculator and recorder uses a portable computer to accurately clock, calculate, and display speed based on the time a vehicle takes to travel a known length of road.

· Aerial speed Measurement. Officers in light aircraft measure vehicle speed based on the time it takes to travel between two or more pavement markings spaced a known distance apart.

· Variable Speed Limits (VSL). This is a type of Intelligent Transportation System (ITS) that utilizes traffic speed and volume detection, weather information, and road surface condition technology to determine appropriate speeds at which drivers should be traveling, given current roadway and traffic conditions (Robinson, 2000).

· Other Speed Measures. These include electronic roadside signs displaying vehicle speeds or other messages and other types of roadway design measures.

How It Is Implemented

Speed reduction programs are usually implemented by local or state/provincial transportation or law enforcement agencies. Legislative action and additional funding may be required to implement some policy changes or new technologies.

Travel Impacts

Studies indicate the elasticity of vehicle travel with respect to travel time is –0.2 to –0.5 in the short run and –0.7 to –1.0 over the long run, meaning that a 10% reduction in average traffic speeds reduces affected vehicle travel by 2-5% during the first few years, and up to 7-10% over a longer time period (Transport Elasticities). This occurs because motorists generally measure travel in terms of time as well as mileage (for example, a particular destination may be described as “20-minutes from town”) so as traffic speeds increase motorists tend to travel more miles.

A comprehensive speed reduction program that significantly reduces traffic speeds on all roads can reduce automobile travel, encourage use of alternative modes (particularly walking and cycling), and help create more Accessible land use patterns, but in practice most speed reduction programs have relatively modest impacts because they apply only to a specific area and so only affect the portion of total vehicle travel. A Traffic Calming program or reduction in posted speeds on a few roads may shift travel to other areas rather than reduce total vehicle travel.

Table 1 Travel Impact Summary

Objective	Rating	Comments
Reduces total traffic.	1
Reduces peak period traffic.	1
Shifts peak to off-peak periods.	0
Shifts automobile travel to alternative modes.	1
Improves access, reduces the need for travel.	1
Increased ridesharing.	1
Increased public transit.	1
Increased cycling.	2
Increased walking.	2
Increased Telework.	1
Reduced freight traffic.	1

Rating from 3 (very beneficial) to –3 (very harmful). A 0 indicates no impact or mixed impacts.

Benefits and Costs

Benefits include increased Safety, reduced total vehicle travel, improved Walking and Cycling conditions, and environmental benefits. Speed Reductions can improve urban Livability, which encourages more efficient Land Use patterns (DfT, 2004).

The Power Model states that a given relative change in the mean speed of traffic is associated with a relative change in the number of accidents or accident victims by means of a power (exponential) function (Elvik, 2005). This indicates that a 10% change in the mean speed of traffic is likely to have a greater impact on traffic fatalities than a 10% change in traffic volume. Speed is likely to be the single most important determinant of the number of traffic fatalities. According to the Insurance Institute for Highway Safety (IIHS, 2003), motorists are driving faster in the U.S., with the result being increased automobile deaths and injuries. One major study concluded that speeding is the number one road safety problem in many countries, and that reducing average speeds on the roads by only 5% will save around 20% of current fatalities (OECD/ECMT, 2006).

Even modest speed reductions can prevent many collisions, and reduce the severity of damages and injuries that result when crashes occur, and are particularly effective at reducing injuries to pedestrians and cyclists (Kloeden, et al., 1997; Leaf and Preusser, 1998; Stuster and Zail Coffman, 1998; IIHS, 2000; Elvik 2001; Kloeden, McLean, and Ponte, 2001; Gårder, 2004; Racioppi, et al., 2004). Taylor, et al (2000) estimate that each 1 mph reduction in average traffic provides the following reductions in vehicle accidents:

• 6% for urban main roads and residential roads with low average speeds.
• 4% for medium speed urban roads and lower speed rural main roads.
• 3% for the higher speed urban roads and rural single carriageway main roads.

Since roadway traffic capacity is maximized at 30-45 mph, speed reductions can increase traffic flow and reduce traffic Congestion delays. Reducing traffic speeds to the 20-40 mph range tends to reduce vehicle operating costs such as vehicle wear, Energy Consumption and Pollution Emissions. Speed reductions reduce traffic noise, particularly if traffic flow is smoothed to reduce hard accelerations. Traffic speeds below about 20 mph may increase per-mile fuel consumption and emissions. Speed reductions that result in smoother traffic flow also reduce vehicle costs and emissions, while those that involve increased stopping may increase these impacts.

Costs include program implementation and operations, and reduced mobility for motorists. Speed law enforcement programs may be self-funding through fines.

Table 2 Benefit Summary

Objective	Rating	Comments
Congestion Reduction	2	Benefits are greatest for reductions from high to medium speeds.
Road & Parking Savings	0
Consumer Savings	1	Benefits are greatest for reductions from high to medium speeds.
Transport Choice	1	Can improve nonmotorized travel conditions.
Road Safety	3
Environmental Protection	2	Benefits are greatest for reductions from high to medium speeds.
Efficient Land Use	1
Community Livability	3

Rating from 3 (very beneficial) to –3 (very harmful). A 0 indicates no impact or mixed impacts.

Equity Impacts

Speed reductions tend to have mixed equity impacts. Motorists often argue that they were unfairly selected for a citation, or that speeds are unreasonably low on a particular road. They tend to reduce external costs of driving (crash risk, air, noise pollution and congestion delays), and benefit people who are transportation disadvantaged by improving walking and cycling conditions. These impacts vary depending on specific conditions.

Table 3 Equity Summary

Criteria	Rating	Comments
Treats everybody equally.	0
Individuals bear the costs they impose.	2
Progressive with respect to income.	0
Benefits transportation disadvantaged.	2
Improves basic mobility.	1

Rating from 3 (very beneficial) to –3 (very harmful). A 0 indicates no impact or mixed impacts.

Applications

Speed reductions can be implemented in virtually any geographic conditions. Federal and state policies can affect speeds on major highways. Regional and local governments tend to apply speed reduction programs on surface streets.

Table 4 Application Summary

Geographic	Rating	Organization	Rating
Large urban region.	2	Federal government.	1
High-density, urban.	2	State/provincial government.	2
Medium-density, urban/suburban.	2	Regional government.	3
Town.	2	Municipal/local government.	3
Low-density, rural.	2	Business Associations/TMA.	1
Commercial center.	2	Individual business.	1
Residential neighborhood.	2	Developer.	1
Resort/recreation area.	2	Neighborhood association.	2
College/university communities.	2	Campus.	2

Ratings range from 0 (not appropriate) to 3 (very appropriate).

Category

Incentive to Reduce Vehicle Use

Relationships With Other TDM Strategies

Speed Reductions support and is supported by Traffic Calming, Vehicle Restrictions, Smart Growth, New Urbanism, School Transport Management, Safety Evaluation and Campus Transportation Management.

Stakeholders

Major stakeholders include transportation planners and engineers, law enforcement officials, motorists and citizens.

Barriers To Implementation

Barriers include resistance by motorists, and technical difficulties implementing changes in road design or increased enforcement.

Best Practices

Traffic speed reductions should be implemented in a comprehensive program that includes Traffic Calming and roadway design, reduced speed limits, driver education and improved enforcement. For information on speed reduction programs see NHTSA and IIHS websites.

Wit and Humor

A police office pulls over a car going much slower than other traffic.

The police asks, “This is a 65 mph highway – why are you going so slow?”

The driver replies, “Officer, I saw a lot of signs that said 22, not 65.”

“Oh, that’s not the speed limit, that’s the number of the highway you’re on!” explains the officer.

The driver replies, “Oh! Silly me! Thanks for letting me know. I’ll be more careful.”

At this point the cop looks in the backseat where two passengers are shaking and trembling.

The officer asks, “Excuse me, what’s wrong with your friends back there? They’re shaking something terrible.”

“Oh, we just got off of highway 119,” explains the driver.

Examples and Case Studies

Pedestrian Safety Operations Proving Effective Pedestrian Safety Operations Proving Effective (http://www.odot.state.or.us/comm/news/2002071801.htm)

Oregon cities have been able to reduce pedestrian crashes by increasing pedestrian law enforcement. Under the Pedestrian Safety Operations (PSE) program, a decoy police officer attempts to cross in a crosswalk, with a video camera recording the event. If passing motorists fail to stop and yield as required by law, they are issued either a warning or a citation. Three years since the program was established crosswalk pedestrian injuries declined by 16% (from 348 to 293) and fatalities declined 19% (from 16 to 13).

Some people have criticized these as “sting” operations, but the program is not designed to surprise or entrap motorists. The purpose is to raise awareness, not write citations. Advance warning is provided through media coverage and on-site signs. Police support the program as an effective crash prevention strategy, with 31 police departments and sheriff’s offices participating in 2002.

“We are grateful to all participating law enforcement agencies. They’ve done a great job,” said Rick Waring, Pedestrian Safety Program coordinator for ODOT. “Pedestrian safety is a serious issue in every community—people have trouble getting across their streets and they are delighted someone is doing something about it. Community response from citizens and public officials has been overwhelmingly positive,” said Waring.

ODOT’s pedestrian safety program also has provided specialized training for 71 police agencies and 108 officers and deputies. The goal is to teach officers to set up the operation so it is fair to motorists, yet has the desired effect of raising awareness and improving safety for pedestrians. For more information, contact the Oregon Department of Transportation Bicycle and Pedestrian Safety Program, (www.odot.state.or.us).

Narrower Streets Reduce Traffic Speeds and Increase Neighborhood Livability (www.ci.vancouver.wa.us/transportation/ntmp/seindex.html)

The Southeast Vancouver Neighborhood Traffic Management program in the city of Vancouver, Washington was initiated in 2002. Similar to many traffic calming exercises, it was intended to build a framework for enhanced livability in an area of Vancouver, Washington that had a tremendous range of street facilities. (The City of Vancouver is located 10 miles north of Portland, Oregon in Washington State.) In the southeast quadrant of Vancouver there are neighborhoods that are tailor-made for walking – combinations of narrow streets, traffic calming measures, trails and sidewalks with landscape strips – a design topology that encourages use of the street by residents. These enclaves of community-sensitive design are commonly surrounded by areas developed primarily in the 1970’s to 1980’s. In many of these older neighborhoods the street design appeared to be guided by a bigger is better perspective, where pedestrian facilities were not even contemplated. This range of street facilities provided a fertile test bed for research into how people perceived their streets and use them.

Analysis and surveys conducted for this program provide insights as to the dichotomy of perspectives individuals have for their street – as a traveler (a driver’s perception of a wide street as convenient or safe) and as a resident (a pedestrian’s perception of higher vehicle speeds as unsafe and undesirable). Based upon analysis of responses to interview surveys, many residents that thought their street was safe for one purpose (driving), also did not conduct activities on their street which would validate its safety (cross the street, have kids play in the front yard, walk). The survey produced consistent relationships – as speed, volume and street width increased, factors that influence livability (such as walking) decreased.

In evaluating the relationship between street width and motor vehicle speed it was found that given a consistent set of street topology characteristics, as street width increases, vehicle speed increases. The relationship was linear and basically suggests that for every 1m [3 to 4 feet] of roadway width, vehicle speeds incrementally increase 1.6 kilometers per hour [1 mile per hour]. However, more significantly the number of vehicles traveling 8 and 16 kph [5 and 10 mph] or more over the posted speed increase geometrically with street width. These higher speed vehicles are commonly the vehicles that place pedestrians at significant risk of injury or death and are the general stimulus that generates calls to city staff with complaints about drivers speeding (an indicator of lower livability). Based upon these findings, the range of 24 to 32 feet width streets appear to produce the most desirable balance of safety, pedestrian access, and vehicle maneuverability. These width findings are consistent with historical standards (1920’s) and Appleyard’s work. Had developers and jurisdictions used these guidelines, many of today’s street livability problems could have been avoided.

While there are numerous traffic calming measures to manage driver behavior and vehicle speed in neighborhoods, there are fewer tools available to address vehicle volume. The same surveys of residents indicated that the greatest level of street activity (including walking) occurred with street volumes less than 1,000 vehicles per day. While vehicle activity can create a sense of security, too much of a good thing seems to have the opposite effect –reduced pedestrian and resident street activity. A second analysis approach was utilized in Vancouver to test the relationship between neighborhood traffic volumes and connectivity, on the assumption that connectivity could provide the basis for reduced street volumes by better distribution or dispersion of traffic. A detailed EMME/2 travel demand forecast model was used to test connectivity options and their influence on neighborhood livability. It was found that a dense grid-like street network was not necessarily needed to provide the benefit of connectivity – but linkages about every 152 to 305 meters [500 to 1000 feet]. However, connectivity by itself was not a panacea. By combining both traffic calming and connectivity, desirable levels of motor vehicle traffic could be achieved and with less extensive neighborhood traffic management than traffic calming alone.

Simply narrowing streets and installing vertical or horizontal deflection traffic calming devices will not assure a livable pedestrian environment. The design of streets should consider street anatomy from a pedestrian perspective: the walls, the border area, and the crossing area. Key tools to establishing a livable walking environment in the pedestrian areas include providing adequate buffer areas, walking zones, driveway and curb ramps, and crossing treatments. These tools can be applied within standard residential street right-of-way widths (ranging from 48 feet to 56 feet) if street curb-to-curb widths are limited to the recommended 24 to 32 feet. Without these key characteristics, pedestrians can be obstructed and forced to travel in the roadway area.

Traffic Taming in Brasília (www.movingtheeconomy.ca/cs_brasilia.html)

Traffic Taming in Brasília was the first Brazilian experience in the control of traffic violence. In the city of Brasília, the average vehicle speed was 90 kilometres per hour and there were 3 to 4 traffic deaths each day, almost half of them pedestrians. In response to public demand, the Government of the Federal District implemented a traffic safety program in 1995 that included the installation of 330 electronic speed-controlling devices, engineering work in critical locations, and the creation of a Traffic Division with 500 police officers.

Electronic speed-controlling devices had been used previously elsewhere but had met with strong resistance from the elite. In Brasília, resistance was minimized through the participation of the local media, which made the subject of traffic a priority. Correio Braziliense, which enjoys 90 per cent of the city's newspaper readership, published daily half-page traffic reports at no cost. The electronic speed-controlling devices were also installed and maintained at no cost by private companies, who received payment from the fines applied.

This program succeeded in creating a revolution in attitudes towards citizenship rights in traffic. This resulted in traffic speed reduction and respect for crosswalks, and a consequent reduction in traffic-related deaths. It created an innovative partnership between media, government and the public. This program acted in areas such as health, traffic, transportation, education, construction, finance and culture, and resulted in improved safety for cyclists and pedestrians. It has since been successfully copied in numerous other Brazilian cities, with significant decreases in traffic mortalities.

References And Resources For More Information

Astrid Helene Amundsen and Arild Ragnøy (2002), Lower Speed Limits On Arterial Roads In Oslo During Winter Season. Effects On Emission, Noise, Traffic And Safety, Norwegian Institute of Transport Economics, Report No. 560/2001 (www.toi.no).

Dan Burden and Peter Lagerway (1999), Road Diets Free Millions for New Investment, Walkable Communities (www.walkable.org). This discusses Traffic Calming projects on arterials.

CORDIS (1999), Best Practice to Promote Cycling and Walking and How to Substitute Short Car Trips by Cycling and Walking, CORDIS Transport RTD Program, European Union (www.cordis.lu/transport/src/adonisrep.htm).

Coulter Transportation Consulting (2004), Neighborhood Traffic Management in DuPage County: Recommended Actions, DuPage County Mayors and Managers Conference

(www.dmmc-cog.org/pdf-files/TrafficMgmt.pdf).

Adrian Davis (2001), Killing Speed: A Good Practice Guide to Speed Management, Slower Speeds Initiative (http://slower-speeds.org.uk/kscontents.htm).

DEA (1999), Main Street…When a Highway Runs Through It, Transportation and Growth Management Program, Oregon DOT and Dept. of Environmental Quality (www.lcd.state.or.us/tgm/publications.htm).

DfT (2004), New Directions In Speed Management: A Review Of Policy, UK Department for Transport (www.dft.gov.uk/stellent/groups/dft_rdsafety/documents/page/dft_rdsafety_504682.hcsp).

DKS Associates (2002), Vancouver Traffic Management Plan: Street Design to Serve Both Pedestrians and Drivers, City of Vancouver, Washington (www.ci.vancouver.wa.us/transportation/ntmp/seindex.html).

EcoDrive (www.ecodrive.org) promotes safer and more energy efficient driving styles.

Rune Elvik (2001), “Zero Killed in Traffic – from Vision to Implementation,” Nordic Road & Transport Research, No. 1 (www.vti.se/nordic/1-01mapp/toi1.htm).

Rune Elvik (2005), Speed And Road Safety: Synthesis Of Evidence From Evaluation Studies, Transportation Research Board 84^th Annual Meeting (www.trb.org), 2005.

Fehr & Peers and Reid Ewing (2002), Traffic Calming Guidelines, City of Sacramento (www.pwsacramento.com/traffic/ntmp.html).

FHWA (1997), Flexibility in Highway Design, FHWA (www.fhwa.dot.gov).

FHWA, Traffic Calming Website (www.fhwa.dot.gov/environment/tcalm/index.htm) by the Federal Highway Administration provides a variety of resources for traffic calming planning.

B.N. Fildes and S.J. Lee (1993), The Speed Review: Road Environment, Behaviour, Speed Limits, Enforcement And Crashes, Accident Research Centre, Monash University (www.monash.edu.au/muarc/reports/atsb127.html).

B. Fildes, S. Godley, T. Triggs and J. Jarvis (1999), Perceptual Countermeasures: Experimental Research, Monash University (www.monash.edu.au) for the Australian Transport Safety Bureau (www.monash.edu.au).

Kay Fitzpatrick, et al. (2000), Design Factors that Affect Driver Speed on Suburban Arterials, Texas Transportation Institute (http://tti.tamu.edu/product/catalog/reports/1769-S.pdf).

Per E. Gårder (2004), “The Impact of Speed and Other Variables on Pedestrian Safety in Maine,” Accident Analysis & Prevention, Volume 36, Issue 4 (www.elsevier.com/locate/aap), July 2004, pp. 533-542.

IACP (2001), Traffic Safety in the New Millennium: Strategies for Law Enforcement; A Planning Guide for Law Enforcement Executives, Administrators and Managers, NHTSA, USDOT (www.nhtsa.dot.gov/people/injury/enforce/Millennium/index.htm) DOT HS 809 158.

IIHS (2000), Speed Law Enforcement (www.hwysafety.org/safety_5Ffacts/qanda/speed_5Flawenf.htm) and Speed and Speed Limits (www.hwysafety.org/safety_facts/qanda/speed_limits.htm), Insurance Institute for Highway Safety (www.hwysafety.org).

IIHS (2003), IIHS Status Report, Special Issue: Speeding, Insurance Institute for Highway Safety (www.hwysafety.org/srpdfs/sr3810.pdf).

Institute of Transportation Engineers Traffic Calming Information (www.ite.org/traffic/index.htm) provides a variety of information on traffic calming research and resources.

ITE, Manual on Uniform Traffic Control Devices (MUTCD) (www.ohs.fhwa.dot.gov/devices/mutcd.html) defines standard traffic signs and other traffic controls.

Ali Kamyb, et al (2003), “Methods for Reducing Traffic Speed in High-Pedestrian Rural Areas,” Transportation Research Record 1828, TRB (www.trb.org), pp. 31-37.

C.N. Kloeden, A.J. McLean, V.M. Moore and G. Ponte (1997), Travelling Speed and the Risk of Crash Involvement, Federal Office of Road Safety, CR 172 (www.atsb.gov.au/road/res-exec/cr172ex.cfm and http://casr.adelaide.edu.au/speed).

C.N. Kloeden, A.J. McLean, and G. Ponte (2001), Travelling Speed and the Risk of Crash Involvement On Rural Roads, Federal Office of Road Safety, CR 204 (http://www.atsb.gov.au/road/rpts/cr204/index.cfm).

W.A. Leaf and D.F. Preusser (1998), Literature Review on Vehicle Travel Speeds and Pedestrian Injuries, National Highway Traffic Safety Administration, USDOT (www.nhtsa.gov/people/injury/research/pub/hs809012.html).

Hans Peter Lindenmann (2005), “Effects on Road Safety of 20 Kilometer-Per-Hour Zone Signposting in Residential Districts,” ITE Journal, Vol. 75, No. 6 (www.ite.org), June 2005, pp. 50-55.

Peter Swift (1998), Residential Street Typology and Injury Accident Frequency, Swift and Associates (www.sierraclub.org/sprawl/transportation/narrow.asp).

Eric Meyer (2001), “A New Look At Optical Speed Bars,” ITE Journal, Vol. 71, No. 11

(www.ite.org), Nov. 2001, pp. 44-48.

National Highway Traffic Safety Administration (www.nhtsa.dot.gov) provides information on the risks of speeding, safety benefits of speed reductions, and recommendations for traffic speed reduction strategies, such as “Traffic Law Enforcement Programs” (www.nhtsa.dot.gov/people/injury/enforce).

OECD/ECMT (2006), Speed Management, Joint Transport Research Centre of the Organisation for Economic Co-operation and Development and the European Conference of Ministers of Transport (www.cemt.org/JTRC/WorkingGroups/SpeedManagement/index.htm).

Ottawa (2004), Area Traffic Management Guidelines; Appendices (Draft), Department of Public Works and Services City of Ottawa (www.ottawa.ca); available at http://ottawa.ca/calendar/ottawa/citycouncil/trc/2004/10-20/ACS2004-TUP-TRF-0012%20Annex%202.pdf and http://ottawa.ca/calendar/ottawa/citycouncil/trc/2004/10-20/ACS2004-TUP-TRF-0012%20Appendix%20A-H.pdf.

Francesca Racioppi, Lars Eriksson, Claes Tingvall and Andres Villaveces (2004), Preventing Road Traffic Injury: A Public Health Perspective For Europe, World Health Organization, Regional Office for Europe (www.euro.who.int/document/E82659.pdf).

Elihu D. Richter, Tamar Berman, Lee Friedman and Gerald Ben-David (2006), “Speed, Road Injury and Public Health,” Annual Review of Public Health, Vol. 27 (http://arjournals.annualreviews.org, April 2006, pp. 125-152.

Mark Robinson (2000), Examples of Variable Speed Limit Applications, Transportation Research Bo