Transactions on the Built Environment vol 23, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509

The environmental impacts of traffic management
schemes
J.Cloke,R.E.Layfield

Abstract
The primary concerns of traffic management are to reduce congestion and/or
accidents. However traffic management schemes will have an environmental
impact, either beneficial or adverse, as a result of the changes in driver
behaviour. This in turn has an impact on the operation of vehicles and hence
exhaust emissions, noise and vibration. In addition the public's perception of
traffic management schemes may be influenced by factors such as disruption
encountered during the construction phase, visual intrusion caused by, forexample, extra traffic signs, severance and perceived danger.
Schemes which reduce congestion can lead to a reduction in the number ofstop-start journeys and a corresponding reduction in fuel consumption andvehicle emissions. However, the relief of congestion could lead to trip
generation, cancelling out any benefits gained by increasing the average speed
through an area.
Accident reduction schemes generally have the aim of reducing traffic speeds
which may lead to increased fuel consumption and vehicle exhaust emissions.
Noise levels of light traffic may be reduced, but there is evidence to suggestthat noise from heavy vehicles may increase for certain traffic calming
measures such as road humps.
There is a need to examine in more detail the environmental implications oftraffic management schemes, given that local and central government face adilemma over the increasing congestion in our towns and cities and thesubsequent potential for environmental degradation. This paper describes thefindings of preliminary research by TRL, commissioned by the Driver
Information and Traffic Management Division of the Department of Transport,of the likely environmental benefits and disbenefits of traffic management
schemes and describes in detail TRL's continuing research programme in thisarea.

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Introduction

Road transport is highly valued both by individuals, who spend a significant
amount of their income on the purchase and use of cars, and industry where
the transport of goods on an efficient road network is vital for national andinternational trade. However, the widespread and escalating existence of roadtraffic is increasingly recognised as a major contributor to a range ofenvironmental impacts. For example urban air quality is causing publicconcern largely as a result of vehicle emissions, with road traffic now thelargest source of urban pollutants in the UK. For many compounds the roadtraffic contribution makes other sources insignificant as can be seen in Table
1.
Table 1 Sources of principal air pollutants in London (1991)*.
Percentage of total emissions
Source ofemission

Carbondioxide

Black
smoke

Road transport

33

96

99

76

97

Other transport
Electricity supply
industry

3
2

2

1
0

4
1

1
0

Other industry

13
30

1

5
6
8

1
1

19

0
2

0
0
0

0

8508

19

648

137

116

Domestic
Other
Total (kt/yr)

0

Carbon Nitrogenmonoxide oxides

Volatileorganic
compounds

Note: percentages do not always total 100 because of rounding
The adverse impacts of road traffic can be minimised in three principal ways:
changes in vehicle technology such as the introduction of catalytic converters
to reduce emissions from an individual vehicle; modifying vehicle operation
characteristics such as reducing congestion in order to reduce emissions andfuel consumption during a journey, and; modification of transport demand to
reduce the amount of travel undertaken in private vehicles.
Traffic management is the general term given to schemes which control theoperation of vehicles and the demand for transport. This paper briefly
describes a review carried out by the Transport Research Laboratory ,commissioned by the Driver Information and Traffic Management Division of
the Department of Transport, on how traffic management might minimise theTransactions on the Built Environment vol 23, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509

Urban Transport and the Environment 211
impact on the environment, in particular on urban air quality. The detailed
findings and a full reference list are given in Abbott et aP.
2

The Impact of Traffic Management Schemes on Urban
Air Quality

2.1

Background

The objectives of traffic management are usually a reduction in congestion
and/or an improvement in safety. Schemes have generally been assessed interms of their success in optimising journey time, controlling traffic speeds
and flows and reducing accidents. More recently engineers and planners havebecome interested in the environmental consequences of traffic as a criterion
in the design of control methods, but more often than not this has been of
secondary importance. As traffic in our towns and cities grows, environmental
degradation will also increase and there is a need to know how traffic
management can contribute to minimising the impact on the environment.
2.2

Urban Traffic Control

Urban traffic control (UTC) is the optimisation of traffic light control systems
to minimise delays in relation to expected traffic flows. Developed in the
1960s, the earlier systems such as TRANSYT were fixed and could not
respond to real time changes in flow, but needed to be updated on a regularbasis. The more modern systems such as SCOOT optimise traffic lights inrelation to actual flows. Smoother flows can lead to reductions in fuelconsumption and therefore vehicle emissions. Robertson et aP has shown that
TRANSYT can achieve savings in fuel consumption by up to 15%, whilst
Hunt et al* and Mulroy* have calculated savings due to SCOOT of 5-10% on
top of that. Provided extra traffic does not take advantage of the reduced
congestion, the implementation of UTC systems can result in reductions infuel consumption, and hence vehicle emissions, within the area covered by the
traffic signal network, but the amount of saving depends on the degree ofsignal coordination already present. The information on change in vehicle
emissions is less well known, with changes in CO and HC likely to be larger
than changes in NO*. The emission reductions relate to the roads covered by
the UTC network so that citywide, the emission benefits may be smaller inpercentage terms. UTC systems can also be used as described above to givepriority to public transport vehicles and to achieve area-wide traffic restraint*.
2.3

Traffic restraint

Localised restraints, such as pedestrianisation, have been very successful inmany cities where their application has typically focused on historic cities,
busy commercial centres and sensitive residential neighbourhoods. In some

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circumstances an area ban may cause a significant degree of inconvenience totraffic and result in the total volume being reduced, and this effect can beenhanced if accompanied by improvements in public transport services andimproved access to walking and cycling^.
Area traffic bans can have a dramatic effect on the traffic levels within thebanned area and are therefore likely to considerably reduce vehicle emissions
within the banned area. However traffic flows outside the banned area may
increase with a consequent deterioration in their environment. Citywide theeffect on vehicle emissions is likely to be small unless the banned area is
extensive.
An alternate number plate scheme and city centre traffic ban has been in
operation in Turin during the winter months. Instead of obtaining an estimated
20% reduction in car use, only 10% is achieved. The relief of congestion has
encouraged people who don't normally drive to use their cars, and it is also
not uncommon for people to buy a second car with the alternate number plate.
Away from the centre, congestion has become unpredictable as drivers try totake advantage of fluctuations in congestion*.
Parking and stopping controls on major urban routes (eg 'Red Routes') can
reduce vehicle congestion, increase speeds and lead to a moderate reductionin emissions, assuming that no additional traffic is drawn onto the routes*.
The availability of parking spaces has a major influence on the choice ofmeans of transport. Parking controls such as increasing charges or reducing
the number of spaces can greatly affect car use within the central area.
However, the implementation of these measures is hindered by the availabilityof private non-residential parking and the need to ensure that economic
activity is not depressed by car users transferring to another area whereparking is cheaper or more readily available.
Changes to emissions citywide as a result of parking controls are likely to berelatively small, at about 2 to 6 per cent, and will depend on the relative
contribution that is made by such schemes in terms of veh-km travelled within
the citywide network.
2.4

Public Transport, Cycling, Walking and Car Sharing

Bus priority measures such as dedicated lanes are capable of increasing thereliability of services and reducing journey times, and have been shown to
reduce bus emissions by up to 35 %\ Vehicle detection systems can also be
used to give priority to buses at junctions, increasing the average speed ofvehicles along the route and reducing emissions by a further 15 to 30%*°.
However observations suggest that the average speed of the rest of the traffic
may suffer and off-set any environmental benefits achieved by the buses. The

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Urban Transport and the Environment 213
overall effect of bus priority measures on vehicle emissions has not been
quantified but is likely to be small unless the priority measures greatly
increase congestion for non-bus traffic^.
Park and ride schemes which provide parking on the fringes of urban areas orcity centres and served by public transport may achieve congestion reduction.
Park and ride has the potential to reduce car use within the inner city area but
the extent to which this is achieved has generally not been well documented.
A study of park and ride schemes in several UK cities** has however
suggested that additional trips are generated and drivers travel further
distances. Some form of car restraint for example central area parking control,
is likely to be required to encourage drivers to transfer to park and ride fromcar use and prevent any reduction in congestion from being offset bysuppressed demand. Similarly new/expanded mass transit systems such as lightrailways, have the potential to reduce vehicle emissions by reducing thevolume of road traffic and congestion. This is not always achieved and someform of car restraint is needed to encourage the transfer.
The promotion of cycling and walking has the potential to reduce the use of
motorised transport in urban areas and thereby help improve the environment.
A national travel survey^ found that of all journeys in Great Britain, 73 percent were less than five miles and 29 per cent were less than 1 mile. The car
was used for 45 per cent of journeys less than 5 miles long and walking
accounted for 81 per cent of all journeys less than 1 mile. Shorter trips tend
to generate higher emissions due to the greater level of energy consumption
when driving with a cold engine. Increasing the number of short journeys
undertaken by bicycle or on foot could therefore have a disproportionately
beneficial impact on energy consumption and emissions. However, theprovision of improved cycling and pedestrian facilities alone is not likely tohave a large impact on their use. To initiate changes in mode the "community
climate" needs to be favourable and cycling and pedestrian facilities need to
be integrated into a comprehensive traffic management plan. In the UK a
strategy for encouraging increased use of bicycles for all age groups is being
developed. This recognises the part which cycling can play as a genuine modeof transport for local journeys within a wider framework of establishingsustainable transport options.
Finally for the motorist very reluctant to travel with a large number of people,
car sharing is an option. The potential benefits of car sharing are clear: where
average occupancies are 1.5 or less (typical for European and North American
cities), filling all car seats with other car drivers could reduce vehicle exhaustemissions by up to 80% as a result of far fewer vehicle-km travelled and thevirtual elimination of congestion^. The provision of high occupancy vehicle(HOV) lanes can reduce travel time and increase travel reliability and, at somelocations, increase car occupancy to about 2 persons per vehicle. The realeffect of car-sharing on emissions is however uncertain as some car passengers

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Urban Transport and the Environment

may transfer from public transport.
2.5

Financial Measures

Financial disincentives to bring about changes in transport modes and/or travel
periods may be temporary measures intended to encourage first-time use or
more permanent measures designed to encourage their continuous use. Tolls
and area licensing schemes, where permits are required for travel in certain
zones, can produce large reductions in car trips and vehicle kilometres within
the cordoned area, depending on the level of the charge, but car trips in theouter area are likely to increase (OECD^). The reduction in vehicle emissions
within the cordoned area may be substantial (about 10 to 40 per cent) but theoverall city wide reduction in emissions are likely to be smaller at about 5 percent. In order for the cordon charges to be successful, other travel modes
must be available to encourage transfer from car use, for example byfinancing improvements in bus services and bus priorities and improved
accessibility to walking and cycling.
Subsidised fares may be used to increase the attractiveness of public transport
and increase bus patronage. However, it is likely that this will lead to only
modest environmental improvements, as studies have shown that althoughhalving bus fares pushes up the occupancy, this is at the expense of walkers
rather than car users. The additional buses required may also offset any
emissions savings due to the small number of car drivers switching mode.
2.6

Lower Speed Limits and Traffic Calming

The introduction of 20 mph (30 km/h) speed limits and zones including
physical traffic calming measures usually reduces NO* emissions on theaffected roads (eg Sammer" and GFMPT '*). However, the effect of themeasures on fuel consumption and emissions of CO and HC are less certain
and may lead to an increase. To achieve a general reduction in emissions,traffic calming schemes require a road design that allows and encourages
smooth driving behaviour. On individual streets, possible increases in vehicle
emissions due to traffic calming may be offset by lower traffic volumes.
Because the residential network carries a small proportion of the total traffic,changes in whole-town vehicle emissions due to residential traffic calming
schemes are likely to be small. However consideration also needs to be given
to the high emission rates due to cold starts about half of which will beoccurring in residential areas.
3

Summary and Discussion

An indication of the effectiveness of each traffic control measure at reducing
car use, fuel consumption or vehicle emissions is summarised in Table 2. The

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Urban Transport and the Environment 215
changes in car modal split and €83 emissions are based on estimated changes
from a modelling study of the effect of different transport policies on cities
with populations of 180,000 to 500,000'*.
Although hard evidence is scarce, the main conclusion is that whilst someschemes have the potential to reduce vehicle emissions, to achieve an optimum
reduction requires some form of vehicle restraint.
Schemes such as UTC and parking controls on main urban routes have beenshown to reduce emissions and fuel consumption on a local level, but theeffectiveness is not clear as without some form of car restraint the reduced
congestion may attract more vehicles, off-setting any benefits achieved.
On their own, improvements in public transport services, bicycle facilities, carpooling schemes and the provision of park and ride sites, are unlikely to
reduce car use and hence vehicle emissions, unless integrated into a traffic
management plan with some form of car restraint.
Overall it can be concluded that whilst the local or inner city effects ofindividual traffic management measures may be large, the citywide effects will
generally be relatively small (eg 5 per cent reduction in CO2 equivalentemissions). Combined measures may have a greater effect but are unlikely to
produce citywide reductions in CO2 emissions greater than, say, 10 per centunless very substantial restraint measures are used. Significant benefits interms of urban air quality are more likely to be achieved by improvements invehicle emissions control technology.
4

Future Research

The review of traffic management schemes carried out at TRL? has shown that
there is a lot of uncertainty in the impact of different traffic management
schemes on vehicle emissions and urban air quality. In particular little
information exists on the effect of a particular scheme on driver behaviour andthe relationship between driving style and vehicle emissions. TRL are now
involved in the initial stages of research which includes:•

the development of representative driving cycles from before and aftersurveys of traffic flow and speed and the instrumentation of vehicles
to measure engine speed during an actual journey•modelling studies of schemes using the driving cycles developed above
and an emissions database to calculate changes in emissions on botha local and area scale

Transactions on the Built Environment vol 23, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509

Table 2 Summary of the effects of urban transport and traffic management measures on modal split,
fuel consumption or vehicle emissions. (From Abbott et aP).
Urban traffic management
measureEffect on modal split, fuel consumption or vehicle emissions
Outer city/local

Inner city/central area

Citywide

UTC system for central area
traffic signals

Not known

Reduction in fuel consumption
between 5 and 15 % but easiercirculation may attract more
vehicles offsetting benefits

Not knownl»o^Parking control on majorurban roads

Reduction in vehicle
emissions of 1 to 16 % onroutes affected

Reduced congestion may attract
more vehicles

Not known

CD

Doubling parking charges incentral area

Increased car modal split
from 61% to 64%

Reduced car modal split from
56% to 43%

Reduction in CC>2
equivalent emissions
between 2 to 4 %Provision of park and ride

May increase car trip
lengths

May have little impact on car
use without car restraint

Uncertain

Central area traffic ban

Increases on traffic outsidebanned area

Reduction in emissions inproportion to vehicles banned.

Probably quite small<%5%Central area licence/toll

Increased car modal split
from 61% to 65%

Reduced car modal split from
56% to 35%

Reduction in CO?
equivalent emissions
between 4 to 6 %05

Transactions on the Built Environment vol 23, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509

Urban traffic management
measureEffect on modal split, fuel consumption or vehicle emissions
Outer city/local

Inner city /central area

Citywide

Mass transit system

May have little impact on
car use without car restraint

May have little impact on car
use without car restraint

Uncertain

Public Transport Priority

Bus emissions reduced by
up to 60%. May increase
emissions from othervehicles.

Little impact on modal split
without car restraint

Uncertain but
probably smallchanges

Car sharing

HOV lanes can increase
average car occupancy

Little impact on modal split
without car restraint or rideshare legislation. May cause
transfer from public transport

Uncertain but
probably very small

Promotion of cycling andwalking

Has potential but little
impact on modal split
without car restraint

Has potential but little impact
on modal split without car
restraint

Probably very small
reduction

Lower speed limits

Lower exhaust emissions

Lower exhaust emissions

Small changes (egup to 2% reduction
inNOJ

Traffic calming(lower speed limits andphysical calming measures)

Probably reduce NO* but
may increase HC, CO andfuel consumption.

Uncertain but
probably very small

33
3

if

3
CD

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Urban Transport and the Environment•

questionnaire surveys of drivers to determine how different schemes
affect driving patterns and attitudes to travel•

before and after air quality monitoring to investigate changes brought
about by any changes in emissions.

Another aspect of the research project is to investigate the noise and vibration
impact and also public's perception of different schemes.
Acknowledgements
The work described in this paper forms part of a Department of Transport
funded research programme conducted by the Transport Research Laboratory
and is published by permission of Driver Information and Traffic Management
division and the Chief Executive of TRL.
The authors would like to thank Mr J Hickman and Dr P Nelson of TRL andMr E Wyatt of DoT for their comments and assistance in the preparation ofthis paper.
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Copyright Transport Research Laboratory 1996.