This document summarizes a case study presentation on modeling road gradients and cold starts for an air quality assessment. It discusses using terrain data and GIS software to calculate road gradients to estimate their impact on vehicle emissions. It also addresses accounting for cold start emissions from heavy duty vehicles leaving a development, noting guidance to apply them within 2.5 km of travel after starting based on a 5 minute warm-up period. Key steps involved terrain analysis, road network digitization, and input parameters for emissions modeling tools.

1. Early Careers Group Meetup
Case Study: Road gradients and cold starts
Sam Bradley (Arup)
4th February 2020

2. 2
• Proposed use: B8 (storage and distribution) commercial
- Main source of emissions: LGV and HGV traffic
• Local council has four AQMAs for NO2 and PM10
• Response to planning application:
- Consider road gradients
- Consider cold start
Introduction and project background

3. 3
• Why?
- Roadside receptors
- AQMA
- Traffic and congestion concerns i.e. major motorway
- High HDV proportion
- Roads with varying heights
Road gradient emissions

4. 4
Road gradient emissions calculations: guidance

5. 5
Emissions Factor Toolkit v9

6. 6
• HDVs only
• Gradients between 0.1% - 30% can be entered
• Impact on emissions is capped at a 6% gradient
• Left blank = 0% gradient
• Flow direction:
- Up hill
- Down hill
- Two way traffic – EFT assumes 50% up hill and 50% down hill
Emissions Factor Toolkit v9 User Guide

7. 7
LAQM Technical Guidance (TG16)

8. 8
• Up hill gradient can increase power demand of engines, particularly HDVs. Therefore,
increase emissions
• Down hill, “the opposite occurs” (see CERC ADMS-Urban & ADMS-Roads User Group
Meeting, November 2019 presentation1)
LAQM Technical Guidance (TG16)
1 Jamie Clayton (2019) Consideration of HGV emissions on variable road gradients in EFTv9

9. 9
• Roads with gradients less than 2.5% and with approximately equal AADT up-hill and
down-hill = no net change in emissions
• Relationship developed for speeds between 10-48kph (6-30mph)
- Calculations are less accurate outside this range, but typically still useable up to 64kph
(40mph).
LAQM Technical Guidance (TG16)

10. 10
Terrain data
• LIDAR Composite
DTM – 2m
• Source: Defra
Road digitising
• Directional traffic
links
• Up-hill and down-
hill
Gradient data
• Calculation of
gradients:
• Terrain data
• Road geometry
Emission calculation
• Screening roads
• EFT parameters
Road gradient methodology

11. 11
• “LIDAR Composite DTM - 2m” from Defra
• Use Digital Terrain Model (DTM) not Digital Surface Model (DSM)
• DSM – includes objects such as:
- Vehicles, buildings and vegetation
- May not be ground level readings
Terrain data

12. 12
Terrain data
• Calculate altitudes for
each vertex in GIS
• Sense check altitudes:
- Vertex location
- DTM should be ground
level, but might pick up
structures

13. 13
• Tips:
- Split links based on terrain
- Crest of a hill
- Valley
- Split links for directional traffic for
2-way roads
- Up hill v down hill
Road digitising
Vertex/split

14. 14
Altitudes (m)
• Vertices from
GIS
• Minimum (MIN)
• Maximum (MAX)
Road lengths
(m)
• Length of each
model link
Gradient calculation
=
𝑀𝑀𝑀𝑀𝑀𝑀 − 𝑀𝑀𝑀𝑀 𝑀𝑀 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎
𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑙𝑙𝑙𝑙𝑙𝑙 𝑙𝑙𝑙𝑙 𝑙
∗ 100
Gradient (%)

15. 15
• Extract vertices from GIS:
- With heights from DTM data
- With calculated total link lengths
Gradient calculation
Split?

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Model link ID MIN altitude
(m)
MAX altitude
(m)
Link length (m) Gradient (%)
1 3.6 10 100 7.5
2 2.3 10 100 5.0
3 2.5 10 100 2.5
4 9.0 10 100 1.0
Gradient calculation

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EFT and model setup

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• Guidance in TG16:
- Emission factors from NAEI
- Guidance on applying cold start
emissions to car park modelling
Cold starts

19. 19
Cold starts
• Only emissions for cars and
LGVs
• How do you account for
HGVs?
- LGV = HGV?
- Uplift LGV emissions
based engine size?

20. 20
• Local authority: apply cold start emissions along network
- Emission Analytics1 quoted:
“For the majority of vehicles tested by Emissions Analytics, it can take more than five
minutes for after-treatment systems to reach operating temperature.”
Cold starts
1 https://www.emissionsanalytics.com/news/can-driving-styles-prove-the-smarter-route-to-better-fuel-economy-and-emissions-lr5c2

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Cold starts
Distance travelled in 5mins
• Assuming the following:
- Only HDVs leaving site
- Average speed = 30kph
- 5mins of cold start
emissions
- Results in 2.5km
travelled distance
Start

22. 22
• Road gradient emissions:
- Key to calculating gradients is terrain data and digitising roads
• Cold start emissions:
Summary & conclusions
Tables of links with maximum increases in pollutant emissions (g/km/s)
Table of link with maximum increase of NO2 emissions