Clean Air Zones (CAZ) are being implemented in cities across the UK in response to government mandate, but also voluntarily, as a recognised means to significantly reduce transport emissions. By defining a compliance standard for vehicles based on classification and fuel-type, the CAZ mechanism provides a standardised approach to discourage the highest-emitting vehicles from entering a defined cordon or zone. For local authorities considering a CAZ, it is necessary to balance the needs of reduced emissions; both local air quality and GHG, within the cordon and in wider area, as well as economic disparity, driver behavioural response, and aspirations to balance compliance with impacts on human health and ecological sites. We consider how to determine if a CAZ is the most suitable measure and, if so, different types of cordon, and which vehicles to include, or alternative tools that may achieve equivalent outcomes. We use examples from recent projects to highlight how developing a good working relationships between air quality, transport, economic and public health professionals can help anticipate risks and identify solutions. We also discuss the relevance of confidence and uncertainty in modelling, for example relating to verification, fleet projections, or sensitivity testing.

1. Clean Air Zones
Developing & using CAZ to improve air quality
Anna Savage BSc MSc CSci MIAQM
Associate Director, Air Quality and Permitting

2. Developing & using CAZ to improve air quality
Overview of CAZ
Considerations for Modelling
Considerations for Implementation
Lessons Learnt
Summary

3. Overview of CAZ
Definition
CAZ Framework and Standards

4. CAZ Definition
“A Clean Air Zone defines an area where targeted action is taken to improve air quality and
resources are prioritised and coordinated in order to shape the urban environment in a way
that delivers improved health benefits and supports economic growth.” Defra, 2020
Non-charging Clean Air Zones
Geographic areas used as a focus for action to
improve air quality. This action can take a range of
forms including, but not limited to, the use of fines for
access restrictions.
Charging Clean Air Zones
In addition to the above, vehicle owners are required
to pay a charge to enter, or move within, a zone if
they are driving a vehicle that does not meet the
particular standard for their vehicle type in that zone.
ULEZ: Where and when - Transport for London (tfl.gov.uk)

5. CAZ Framework
The UK CAZ framework sets out a consistent approach to be
taken by local authorities to achieve;
a) local growth and ambition;
b) a transition to a local emission economy; and
c) immediate action to improve AQ and health
Many local authorities have conducted CAZ feasibility studies
as part of their mandate to develop Local Plans to comply
with EU Limit Values for NO2.
Local authorities can bid for central government funding when
following the framework

6. CAZ Standards
Class Vehicle type
A Buses, coaches, taxis, private hire vehicles
B Buses, coaches, taxis, private hire vehicles, heavy goods vehicles
C Buses, coaches, taxis, private hire vehicles, heavy goods vehicles, vans, minibuses
D Buses, coaches, taxis, private hire vehicles, heavy goods vehicles, vans, minibuses, cars,
motorcycles (optional)
Vehicle type CAZ minimum standard
Buses, coaches, heavy goods vehicles Euro VI
Vans, minibuses, taxis, private hire vehicles, cars Euro 6 (diesel) and Euro 4 (petrol)
Motorcycles Euro 3

7. Corridor
Gateway
Cordon
Hybrid
CAZ Types

8. CAZ Cities
• London – ULEZ for all vehicles. Expanded to inc N/S Circular in Oct 21
• Birmingham – Class D from June 21
• Bath – Class C from March 21
• Portsmouth - CAZ B from November 21
• Bradford – Class C inc ring road from Jan 22
• Bristol - Class D summer 2022
• Tyneside/Newcastle – Class B delayed
• Sheffield – Class B late 2021/2022
• Manchester- Class C May 2022
• Oxford – Existing city LEZ to be a ZEZ (pilot 2022)
• Leeds – cancelled
• Sefton – considering CAZ A or B

9. Modelling Considerations
Fleet projections
Traffic models
Emissions and air quality modelling
Sensitivity tests

10. Fleet Data
Registered fleet composition may not reflect
journeys:
- Small number of vehicles may be
disproportionately high emitters; e.g. few taxi
and buses record high mileage.
- Data should be based as veh/km not fleet
- Global vs specific fleets per link
- Specific bus and taxi data from registered
vehicles
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
0.900
1.000
Pre-Euro
1
Euro 1 Euro 2 Euro 3 Euro 4 Euro 5 Euro 6 Euro 6c
Petrol Cars Diesel Cars LGV
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
Automatic
Number
Plate
Recognition
Baseline
Enforcement
Monitoring

11. Fleet Projection Uncertainties
• How does fleet retention and turnover respond?
• How are old vehicles distributed in the fleet?
• How is alternative technology being adopted?
• What effect does Covid-19 have on fleet
turnover?
• Driver response to CAZ is unpredictable.
Studies may use national data or data from
small scale stated preference survey
• Alternative technology is not always the best
solution and may take time
• Fleet will improve naturally
Fleets will improve in the
future and CAZ will become
less effective.
AQ is plateauing as new
standards are adopted
Development growth may
exceed fleet improvements so
emissions by increase

12. Traffic Model
Limitations
• Traffic models are traditionally “best” at modelling private cars rather than goods vehicles.
• The existing model may not always be the ‘best’ option!
Extent of model
• Consider impacts outside a cordon e.g. wider CO2 impacts/distributional impacts
Behavioural response
• Buses follow routes and Artic HGVs limited to major routes
• Rigid HGVs tend to be local owner-operated and likely to use smaller roads
• LGVs (goods and services), taxis/PH and cars are complex and tend to follow:
- Modal shift
- Cancelled journey
- Change destination
- Rerouting
• Private cars – how to represent shift to active modes, replacement of vehicles
or non-compliant trips
Level of Resolution
• Strategic model across a highway network Vs local scale for junctions (microsim)
Consider suitability of
model for air quality
purposes and level of
resolution required.
Specify requirements
with the traffic team as
early as possible

13. Emissions/AQ Model
• Source apportionment
• Link based emissions modelling to test scenarios
• Air quality modelling at receptors or grids
• Distributional impacts
The outputs need to consider LAQM,
compliance reporting and health disparity

14. Verification
CAZ studies typically cover large areas, e.g. whole districts and surrounding areas, including strategic
and local routes.
Model verification needs to be considered either;
• Single verification, where statistical analysis can achieve good values.
• Zonal verification due to inherent differences in the character or emissions or dispersion e.g.
- Geographic extents
- Road characters, canyons, surface roughness
Verification should be
planned to match the
model extents and
provide confidence in
the most sensitive
locations to inform
decisions.
Consider:
• Use of detailed / simple canyon modules in ADMS; sensitive to detail, but
will slow run times.
• Presence / contribution of tunnels and point sources
• Effects of variable background contribution; can this be validated at more
than one location?
• Are verification points representative of receptor or compliance locations?
• Is there good confidence in model results at key locations?

15. Sensitivity Testing
Ensure that sensitivity testing is incorporated into the programme
Model accuracies
• Input parameters such as met years, gradients, surface roughness, canyons etc
• Can a simpler method be used; e.g. emissions scaling rather than re-running model
• Post processing parameters such as f-NO2, verification factors
Model elasticity
• How sensitive is the model to applied measure responses, and how reliable are these?
https://www.am-online.com/news/market-insight/2021/04/29/uk-car-manufacturing-s-covid-19-recovery-to-take-at-least-six-months/
Covid-19 tests
• Travel behaviour, e.g. fewer commuter trips or change travel
patterns across the day, traffic on key corridors may ease more
than local roads
• Impact on fleet turnover - new vehicle sales down (March output
down by 20-25% on 5 year average)

16. Considerations for
Implementation
Variations, extents and alternatives
Balancing impacts

17. CAZ Extent
Large
zones must
be a lower
class to be
viable
Small
zones can
be more
effective at
higher class
Consider size of cordon and impacts
outside due to distribution/re-routing
Tyneside route options

18. Variations to CAZ Standard
- C-, excluding taxis
- A+, including bus and taxi, and specific
types of LGV
- The US may need a separate class for
trucks
- Specific vehicle exemptions (e.g. residents)
Class A equivalence may be achieved with a
public / private partnership or agreement of
vehicle turnover e.g. for public transport operators
and taxi licensing
Class B may be achieved with vehicle restrictions;
e.g. restricting HGV access.
Class-D may be achieved with parking restrictions
/ variable paring charges to capture car
destination
Funding the direct upgrade or retrofitting bus
fleets
Alternatives to CAZ
Modification to the CAZ framework or an
alternative measure may be as or more
effective than a class of CAZ.

19. Balancing Impacts
KPIs
Objectives?
Priorities?
Emissions?
Exposure?
EU Limits?
Pollutants?
- AQ Compliance vs LAQM
- Health
- Social groups/deprivation areas
- Politics and Economy
- Climate
Holistic benefits should be
highlighted, but there will be costs

20. Climate
Objectives
CAZ are not
currently used
for GHG
Behaviour
CAZ tend to
redistribute the
fleet:
- overall flows
are unaffected
- CO2 change
marginal
Technology
CAZ may
support
adoption of
zero-tailpipe
emission
technologies
Modal Shift
Ideally CAZ will
support a shift
away from
personal
journeys by car
in favour:
- Public
transport
- Active travel
Implementation
The
implementation
package should
be holistic

21. Barriers
• Not all measures that work conceptually will be feasible for implementation for a variety of reasons
Political Without the political buy in from local Councillors (and other stakeholders)
a measure will not get off the ground
Legal State aid rules may prevent direct central government funding of some
CAZ alternatives
Temporal The measure may not be implementable within the timeframes required to
achieve air quality improvements above those that are expected naturally
Financial There may be too high a cost, either direct or indirect
Practical There may not be the resources to fulfil the measure, regardless of funding
(e.g. not enough Euro VI buses in circulation)
Efficacy The measure may be ‘too effective’ for the level of intervention needed, or
not effective enough
Disbenefits Something that solves a problem in one place but creates one somewhere
else is not desirable

22. Key Lessons Learnt

23. Successes
Clear vision and
understanding of
scope
Clear roles and
responsibilities
across the team
Specific stakeholder engagement
team from early on in process
Buy-in from the Council
departments
Sharing of information
in central location

24. Areas to Improve
Theme Description
Politics Issues managing political engagement. Need a broader representation by
members on board
Timescales Tight timescales posed by JAQU, lack of staff resources to complete
modelling or to make decisions
Quality Assurance Tighter QA processes around modelling, including multiple sensitivity
testing
Continuity Changing guidance and personnel over the course of the project lead to
inefficiencies
Suitability Regional scale modelling does not always lend itself well to targeted hyper
local measures (e.g. infrastructure on specific exceedance links)

25. Future Considerations
Monitoring of CAZ effects
• Changes to measured levels post-
implementation
• Use ANPR data to report on compliance
Modelling of independent schemes in a CAZ city
• Share ANPR data
• Apply compliant fleet inside CAZ
• Consider use of different version of tools e.g. EFTs.
• If non-compliance is predicted, compare change to CAZ
modelling
• Receptor locations may differ

26. Summary

27. Summary
CAZ
• Not always the most technically effective option; e.g. fleet
improvement partnership may be as effective
• May be costly
• Likely to be politically opposition
However…
• Dramatic and significant tool to enforce lower emissions
• Careful design can capture a large proportion of the fleet
• May have wider benefits / detriments outside the zone
Therefore…
• Objectives and KPIs must be agreed at the start
• Opportunities to achieve holistic benefits to health and wellbeing
must be identified and maximised.

28. Thank you.