vvEPA
United States
Environmental Protection
Agency
Air and Radiation
EPA420-S-98-012
July 1998
Office of Mobile Sources
TRAQTechnical Overview
Transportation Air Quality Center
Transportation Control Measures:
Traffic Flow Improvements
EPA's main strategy for addressing the contributions of motor vehicles
to our air quality problems has been to cut the tailpipe emissions for
every mile a vehicle travels. Air quality can also be improved by
changing the way motor vehicles are used—reducing total vehicle miles
traveled at the critical times and places, and reducing the use of highly
polluting operating modes. These alternative approaches, usually
termed Transportation Control Measures (TCMs), have an important
role as both mandatory and optional elements of state plans for
attaining the air quality goals specified in the Clean Air Act. TCMs
encompass a wide variety of goals and methods, from incentives for
increasing vehicle occupancy to shifts in the timing of commuting trips.
This document is one of a series that provides overviews of individual
TCM types, discussing their advantages, disadvantages, and the issues
involved in their implementation.
> Printed on Recycled Paper
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Traffic Flow Improvements
Contents
O Background
© Costs and Benefits
© Implementation
O Recent Examples
© Sources
® On-line Resource
T
raffle flow improvements include a range of strategies that
enhance the capacity and efficiency of a roadway system,
without adding extra lanes or new roads. As roadways become
less congested, congestion-related emissions tend to fall.
However, reducing congestion may encourage motorists to make
more trips causing vehicle miles traveled (VMT) and associated
emissions to rise. These strategies can be grouped into three main
types: traffic signalization, traffic operations, and enforcement
and management. Traffic signalization represents the most
common traffic management technique applied in the U.S.
Traffic signal improvements can include the following:
**• Updating traffic signals to include new, more modern hardware, allowing for the
planning of more sophisticated traffic flow strategies.
*+• Timing traffic signals to correspond to current traffic flows, reducing unnecessary
delays.
**• Coordinating and interconnecting signals to better interface pre-timed and
traffic actuated signals, actively managed timing plans, and master controllers to
minimize the number and frequency of stops necessary at intersections.
*+• Removing signals at intersections no longer requiring signalized stop control to
reduce vehicle delays and unwarranted stops.
Traffic operations describe several types of roadway improvement projects, including:
**• Converting two-way streets to one-way operation to improve corridor travel
times and increase roadway capacity.
**• Restricting left turns on two-way streets as a means of eliminating conflicts with
left turn movements, thereby reducing congestion and delay.
**• Separating turning vehicles from through traffic with continuous median strip
turn lanes.
**• "Channelizing" roadways and intersections (i.e., clearly marking travel lanes and
paths with striping and signage to reduce motorist confusion and uncertainty by
channeling traffic in the proper position of the street) to improve vehicular flow
and capacity.
**• Widening and reconstructing roadways and intersections to reduce bottlenecks
along sections where traffic capacity is below that of the adjacent street (e.g.,
traffic islands, turning lanes, and signage).
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Traffic Flow Improvements Page 2
Finally, several types of programs fall under enforcement and management:
**• Incident management systems, consisting of roving tow or service vehicles,
motorist aid call boxes, incident teams, detectors in the roadway lanes to monitor
traffic volumes, signage systems, traffic operations centers, contingency planning,
and improved information availability to consumers through radio and television.
^ Ramp metering, a technique to improve traffic flow on freeways by using signals
to regulate traffic entering the highway so that it enters only at pre-timed intervals
or at times determined by traffic volumes on the ramp or on the highway.
**• All other enforcement of traffic and parking program regulations necessary
when individuals are required to change or adhere to a particular travel and
parking behavior.
1. Background
Traffic flow improvements have been in existence for decades, growing increasingly
more complex as congestion on U.S. roadways has worsened. One prominent early application
was the Chicago Incident Management Program. The program was initiated in 1960 when the
newly opened Kennedy Expressway began to reach near-capacity volumes during peak periods.
To manage the crisis, the Illinois Department of Transportation assigned twenty people in pick-
up trucks to patrol the expressway during the morning and afternoon peak commuter periods.
The emergency patrol, eventually named the "Minuteman Patrol," was charged with keeping the
Kennedy Expressway open by clearing travel lanes of disabled vehicles. Today the program
employs 60 people, covers 80 miles of the 150-mile expressway system, and operates 24 hours a
day. It has an annual operating budget of $3.5 million funded from state motor fuel taxes.
Also in Chicago, in 1963, the first
metered ramp in the U.S. was installed on the
Eisenhower Expressway. The system featured
a police officer, stationed on the entrance ramp,
who stopped traffic and released vehicles one
at a time at a rate determined by a pilot
detection program. Currently, traffic flow
improvements are widely applied by both state
and municipal transportation agencies,
primarily to reduce congestion and improve travel times at specific locations. Other factors
motivating their implementation include financial difficulties in supporting new major
transportation projects, and the environmental and physical constraints associated with new
infrastructure construction.
Traffic flow improvements have been
in existence for decades, growing
increasingly more complex as
congestion on U.S. roadways has
worsened.
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Traffic Flow Improvements Page 3
2. Costs and Benefits
Traffic flow improvements generally provide a cost-effective method to reduce
congestion, although the effects on vehicular emissions can be difficult to quantify. The cost to
implement traffic flow improvements can range considerably, however. Strategies involving
local traffic operations or signal improvements can be implemented at a low cost. Actions
involving right-of-way acquisition or construction have relatively high capital costs. System-
wide traffic operation improvements that involve many sites can be very expensive to implement.
^ Traffic signalization costs vary depending on the type of improvement and
number of signals affected. Updating a signalized intersection requires a new
traffic controller or traffic control software strategy. Timing plan improvements
entail a labor-intensive data collection effort to determine new signal timings and
subsequent re-timing of signals at each location. Signal coordination and
interconnection requires cable installation, as well as a series of controllers or a
centralized computer-based master control system. To remove signals, a field
survey must be performed to substantiate the elimination of the signals; field work
is also necessary to remove the equipment.
**• Traffic operation improvements include a wide array of actions with varying
costs, although they are typically inexpensive when compared to actions such as
constructing new lanes. Converting streets to one-way operations or
implementing left-turn restrictions at intersections involves installing new signage
and possibly removing or relocating existing signs and traffic signals.
Implementing a continuous left-turn median lane requires new signage and lane
markings and modifications to existing signage and signals. Similarly, improving
the channelizing of a roadway or intersection requires pavement striping,
markings, and signage.
*+• Enforcement and management activities impose capital, operating, and
maintenance costs. A facility enforcement program includes the labor costs
associated with traffic control officers providing patrols and surveillance of the
facility during its operation. A traffic and parking enforcement program requires
meter readers, uniformed police officers, and tow trucks. However, the revenue
generated by the fines issued as part of an enforcement program generally exceed
costs by a factor of at least seven or eight. An incident management system
entails costs for embedded traffic detectors, changeable message signs, closed-
circuit televisions, and some type of central computer control. Metered ramps
require additional signals and signage.
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Traffic Flow Improvements Page 4
Because signal improvements reduce
travel times and stop-and-go driving
conditions, they can produce
measurable reductions in carbon
monoxide and hydrocarbon emissions.
The air quality benefits from these
three types of traffic flow improvements are
difficult to predict. Because signal
improvements reduce travel times and stop-
and-go driving conditions, they can produce
measurable reductions in carbon monoxide
and hydrocarbon emissions. Although
system - or region wide - air quality benefits
are likely to be low, measurable local benefits to air quality and congestion relief can be seen
within downtown and other major activity areas. However, motorists might be diverted from
alternative modes of transportation, or at a minimum, the signal improvements might induce
more and longer trips during peak and off-peak travel periods. The subsequent increase in VMT
along a roadway with improved traffic flow would at least partially offset any short-term air
quality improvements generated by faster, more consistent travel speeds.
Traffic operations are similar to signalization improvements in that they are primarily
orientated towards reducing congestion on local and arterial streets by improving the efficiency
of the system. Their system - or region wide - air quality benefits are probably low. However,
in conjunction with their proven effectiveness to improve traffic bottlenecks and flow, they
probably provide measurable reductions in localized carbon monoxide and hydrocarbon
emissions.
Enforcement and management programs provide a variety of tools that, alone or in
combination with other measures such as traffic operations and signalization improvements, can
provide an additional means to improve
traffic flow conditions, both locally and
Travel speeds can be increased by
installing ramp metering, thereby lowering
emissions, although there could be
increases in localized carbon monoxide
concentrations in the areas of individual
ramps if excessive queues develop.
at the corridor-wide level. Travel speeds
can be increased by installing ramp
metering, thereby lowering emissions,
although there could be increases in
localized carbon monoxide
concentrations in the areas of individual
ramps if excessive queues develop. In
addition, it is important to take into
account the excess emissions that may occur during rapid-ramp acceleration.
Finally, along with the travel speed improvements brought about by ramp metering, there
can be measurable increases in traffic volumes, raising the possibility that metering may induce
additional traffic or increase VMT. Where large-scale metering is implemented and results in
substantial improvements to travel speed, it is likely that some percentage of any additional
traffic or VMT detected would reflect a shift from another mode of travel. This shift would at
least partially offset any air quality improvements generated by faster, more consistent travel
speeds on the highway system.
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Traffic Flow Improvements Page 5
Traffic flow improvements typically are
implemented by city and county public
works departments, with financial
assistance usually provided by state and
federal funding sources.
3. Implementation
Traffic flow improvements typically
are implemented by city and county public
works departments, with financial assistance
usually provided by state and federal funding
sources. Because these actions facilitate
urban driving, they usually generate little
public opposition, with the exception that local residents may object to the disruption caused by
construction and to such actions as street reversals.
Many traffic flow improvements involve changes in the attitude and behavior of local
residents and commuters. As a result, the most successful programs are likely to be those that
provide the greatest incentives or disincentives to change. Strict enforcement of such traffic flow
improvements as restricted left turns and parking limitations, for example, discourages violations
of these measures. With enough effort early-on, the initial level of enforcement can be reduced
later. Overly restrictive measures, however, should be avoided. Very high fines, for instance,
could be considered unacceptable by the majority of users and could foster resentment of the
program.
Factors that can negatively impact the implementation of traffic flow improvements
include the relatively long amount of time necessary to complete some improvements and the
scarcity of available funding. Enforcement and management strategies typically involve a
substantial amount of time and planning to implement. Implementation of highway information
management systems, ramp metering from conceptual planning to a complete system, for
example, may require five to ten years. Traffic signalization and traffic operation improvements
are generally less time-intensive.
Many small jurisdictions and even some large central cities have limited traffic
engineering capabilities and budgets. In these cases, traffic signal management and roadway
maintenance and design are often limited to the most basic or rudimentary installation and
maintenance functions. In addition, state DOTs have a strong influence over the allocation of
federal roadway aid funds. Many states have priorities that stress capital-intensive road and
bridge building rather than traffic operations and signal control systems.
4. Summary of Recent Examples
Traffic flow improvements are becoming increasingly common across the U.S. Ramp
metering programs, for example, are currently operating in 23 metropolitan areas in 18 states.
Since 1989, the number of operating meters in North America has increased from about 1,600 to
over 2,300, an increase of about 45 percent. [1] Two recent examples of comprehensive traffic
flow improvement programs are discussed below.
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Over the past several years, the City of
Sacramento has initiated a series of systematic
programs to improve its signalized traffic
control systems. [2] The programs have
included changing signals that are primarily
located in outlying areas from a system of
interconnected, pre-timed signals to a system
consisting of traffic-actuated signals operated
by computer-based control. To improve the
220 fixed-time signal intersections located mainly within the downtown area, the City received
direct financial and technical assistance through the California Fuel Efficient Traffic Signal
Management program to re-time and optimize the signal phasing to minimize delay and increase
overall arterial travel speeds. A future program is envisioned by the City to install a master
computer network that will allow the traffic system to be more thoroughly monitored and
controlled from a central site.
Traffic flow improvements are
becoming increasingly common across
the U.S. Ramp metering programs, for
example, are currently operating in 23
metropolitan areas in 18 states.
Over the past several years, the City of
Sacramento has initiated a series of
systematic programs to improve its
signalized traffic control systems.
Post-implementation surveys
identified several positive results. The
arterials located in areas outside the
downtown area all experienced reductions
in vehicle delay averaging about five
percent (as measured in vehicle-hours of
travel). These benefits were derived
throughout the day, not just at peak travel periods. For signalized roadways within the
downtown area, the data indicated that the signal timing optimization program has led to an
overall travel-speed improvement often percent, with a comparable reduction in vehicle delay.
Vehicle counts show that the "preferential signal timing" strategy has likely caused traffic to
increase on these roadways, along with a higher travel speed during peak periods. Whether this
induced demand represents trips diverted from other streets or if the additional vehicles represent
new trips was not determined conclusively. Air quality effects were not estimated.
In the fall of 1987, the City of Boston Transportation Department initiated a
comprehensive study of the City's historic Back Bay district to develop a short - and long range
- transportation management policy plan. [2] The plan primarily focused on two major arterials.
The traffic operational measures implemented included the following:
^ Left-turn restrictions at several intersections that currently experience a poor level
of service
^ Left-turn bays at several other locations to reduce conflicts
^ Street widening of a few feet along several segments to gain an additional travel
lane or allow for a left turn lane
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Traffic Flow Improvements Page 7
Management measures that were implemented involved on-street parking and included
establishing new no-stopping zones at selective locations either for the peak period or throughout
the day; relocation and consolidation of cab stands, tour bus stops, loading zones and
handicapped parking spaces; and removal of short-term parking meters. Enforcement activities
featured a highly visible program that included meter readers, motorcycle police officers, and two
trucks.
The benefits derived from the traffic plan on area mobility were significant. Illegal long-
term parking at on-street meters was reduced considerably. Double parking was almost
eliminated. Average travel speeds on the arterials increased from as low as 6 mph to over 12
mph. Travel time reductions of over 30 percent were seen for both roadways. Significantly,
arterial travel times after implementation remained relatively constant throughout the day, rather
than deteriorating significantly by the afternoon peak period, as was the case before the plan was
implemented. These travel time reductions were realized despite traffic counts that indicated the
arterials were carrying 30 to 40 percent more traffic during the peak hours. Analogously, traffic
counts indicated that other roadways, and particularly several streets located in residential areas,
had experienced a corresponding reduction in vehicle use, ranging from five percent to more than
40 percent of the peak hour traffic. The impact of the plan on air quality was not estimated.
6. Sources
[1] U.S. Department of Transportation. Ramp Metering Status in North America (1995 Update).
DOT-T-95-17. Washington, D.C. (June 1995).
[2] U.S. Environmental Protection Agency, Office of Air and Radiation. Transportation Control
Measure Information Documents. 400-R-92-006. Washington, D.C. (March 1992).
7. On-line Resource
The Environmental Protection Agency's Office of Mobile Sources has established the
TCM Program Information Directory to provide commuters, the transportation industry, state and
local governments, and the public with information about TCM programs that are now operating
across the country. This document and additional information on other TCMs and TCM
programs implemented nationwide can be found at:
http://www.epa.gov/omswww/transp/traqtcms.htm
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