NAEI Reference:
ED56186506
Date:
21st January 2014
By:
Yvonne Pang and Tim Murrells (AEA)
TEMPORAL VARIATION IN EMISSIONS FROM ROAD TRANSPORT
This note describes analysis of the temporal variation in emissions from road transport by
time-of-day, day-of-week and month. Profiles describing the relative change in emissions
over these time periods are provided which can be applied to the annual emission rates
given by emission inventories. This is so that hourly emission rates for traffic required for
input into air pollution models can be developed. Temporal emissions profiles were derived
previously in a study by Jenkin et al (2000) which looked at the temporal dependence of
ozone precursor emissions and ozone exceedences.
The new temporal profiles are based on the recent information published by the Department
for Transport (DfT) on temporal variation in traffic flows1. DfT provides the distribution of
traffic flows by month, day of week and time of day as indices relative to average traffic flows
of the time period concerned (represented by a value of 100). The following dataset have
been used in the analysis:




Average daily traffic flow (five year average) by month in Great Britain. This dataset
is broken down by two main vehicle groups (“Cars & taxis” and “Goods vehicles”) and
by motorways, rural and urban roads;
Average traffic distribution by day of the week in Great Britain, with the same
breakdown as above;
Traffic distribution by time of day (Monday to Sunday) on all roads in Great Britain;
Car traffic distribution by time of day (Monday to Sunday) on all roads in Great
Britain;
Temporal variations in hot exhaust emissions can be directly related to the temporal
variation in traffic flows. Cold start and evaporative emissions are also dependent on
ambient temperature and trip characteristics. Diurnal or monthly temperature data are
available from the Met Office, provided as minimum, maximum and mean temperatures.
The proportion of trips started with the engine cold and at what time of the day is also
required. For simplicity, this current analysis has assumed the traffic flows trend applies to
cold start and evaporative emissions. Further work is needed to improve the emission
profiles for cold start and evaporative emissions based on trends in the specific additional
factors that influence these emissions.
The following section discusses the time-resolved traffic flow distribution and provides
illustrations of emissions distribution by month, day of week and time of day respectively
calculated from it.
1
http://www.dft.gov.uk/statistics/series/traffic
An Excel spreadsheet with profiles defining the temporal distribution of emissions is provided
with this note on the NAEI website.
Variation by Month
Figure 1 shows that:

Cars & taxis traffic on urban roads does not vary significantly by month; there are
small dips in traffic during August and December which are likely to be due to
summer/public holidays. Cars & taxis traffic on rural roads and motorways are
generally higher during summer months than in winter months.

Goods vehicles on urban, rural and motorway roads have similar traffic variations by
month, and traffic seems to dip around holiday seasons (e.g. in August and
December).
Figure 1. Average daily traffic flow (2006-2010 average) by month in Great Britain
normalised to the annual average (=100)
The “Cars & Taxis” traffic distribution profile has been used to proportion emissions from
cars and motorcycles by month while the “Goods Vehicles” profile has been applied to
LGVs, HGVs and buses. Table 1 is a summary of the monthly variation in emissions for cars
& taxis and goods vehicles on the different road types.
Table 1: Fraction of Road Transport Emissions Emitted by Month (2006-2010 Average)
Cars & Taxis
Motorways
All rural
major
and
minor
roads
January
0.0735
0.0731
February
0.0767
March
All
urban
major
and
minor
roads
Goods Vehicles
All
All rural
urban
major
major
and
and
minor
minor
roads
roads
All
roads
Motorways
All
roads
0.0801
0.0745
0.0777
0.0756
0.0762
0.0774
0.0760
0.0808
0.0773
0.0809
0.0806
0.0811
0.0809
0.0816
0.0817
0.0851
0.0822
0.0856
0.0848
0.0858
0.0855
April
0.0843
0.0849
0.0839
0.0843
0.0833
0.0837
0.0829
0.0833
May
0.0847
0.0866
0.0844
0.0850
0.0839
0.0842
0.0832
0.0839
June
0.0862
0.0882
0.0854
0.0864
0.0874
0.0883
0.0872
0.0875
July
0.0880
0.0896
0.0850
0.0878
0.0859
0.0866
0.0878
0.0861
August
0.0901
0.0905
0.0821
0.0888
0.0821
0.0843
0.0835
0.0825
September
0.0879
0.0877
0.0850
0.0874
0.0867
0.0882
0.0874
0.0870
October
0.0873
0.0857
0.0849
0.0866
0.0866
0.0874
0.0867
0.0867
November
0.0828
0.0813
0.0843
0.0828
0.0867
0.0856
0.0857
0.0865
December
0.0770
0.0747
0.0789
0.0769
0.0733
0.0706
0.0723
0.0729
Total
1.0000
1.0000
1.0000
1.0000
1.0000
1.0000
1.0000
1.0000
As the variation in “Cars & Taxis” traffic on urban roads is slightly different to that for rural
and motorways, variations in total emissions for each pollutant by month are plotted
separately for these road types as shown in Figure 2 for NOx, PM10 and NMVOCs.
As shown in Figure 2, the profile pattern is affected by the contribution of emissions from the
two vehicle groups (“Cars & Motorcycles” and “LGVs & HDVs”) and by road types. For
instance, the majority of NMVOCs emissions come from Cars & Motorcycle; due to their
higher traffic activity on motorways and rural roads in summer months, NMVOCs emissions
maximise during this period whereas NOx and PM10 emissions do not show such a
significant monthly variation on these road types.
Figure 2. NOx, PM10 and NMVOCs emissions distribution on urban roads (Left) and
motorways and rural roads (Right) by month
Variation by Day of Week
Figure 3 shows that:

Cars & taxis traffic levels are similar each day between Monday and Thursday on all
road types, while traffic levels peak on Friday and fall during the weekend.

Goods vehicle traffic is fairly similar each day between Monday and Friday, but it
drops down markedly during the weekend.
All road types show very similar
distribution of traffic flows by day of the week.
Figure 3. Average traffic distribution by day of the week in Great Britain, 2010
The variation in traffic trends by road types is generally quite small, except car traffic on
motorways increases from Saturday to Sunday (although still lower than the weekday level)
while there is a general fall in traffic for urban and rural roads over the weekend from
Saturday to Sunday.
Table 2 is a summary of the daily variation in emissions for cars & taxis and goods vehicles
on the different road types.
Table 2: Fraction of Road Transport Emissions Emitted by Day of Week (2010)
Cars & Taxis
All
All
rural
urban
major
major
and
and
minor
minor
Motorways
roads
roads
All
roads
Goods Vehicles
All
All
rural
urban
major
major
and
and
minor
minor
Motorways
roads
roads
All
roads
Monday
0.144
0.144
0.144
0.144
0.166
0.168
0.168
0.166
Tuesday
0.141
0.144
0.150
0.143
0.182
0.185
0.182
0.182
Wednesday
0.144
0.145
0.151
0.145
0.187
0.186
0.182
0.186
Thursday
0.148
0.149
0.153
0.149
0.186
0.188
0.183
0.186
Friday
0.160
0.163
0.156
0.160
0.165
0.167
0.171
0.166
Saturday
0.130
0.133
0.134
0.131
0.066
0.063
0.074
0.066
Sunday
0.133
0.121
0.112
0.127
0.048
0.043
0.041
0.047
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
Total
Estimated emissions of each pollutant by day of week are plotted separately for these road
types as shown in Figure 4 for NOx, PM10 and NMVOCs. Emissions from cars and
motorcycles stay at similar levels throughout the week with slightly lower emissions over the
weekend. However, emissions from LGVs and HGVs are noticeably lower over the
weekend due to their lower traffic activity. The drop in total traffic emissions from weekday
to weekend differs for each pollutant because of the different contributions made by each
vehicle type with different daily traffic profiles.
Figure 4. NOx, PM10 and NMVOCs emissions distribution on motorways (Left) and
urban and rural roads (Right) by day of week in 2010
Variation by Hour of Day
DfT provides traffic distributions by time of day for cars and for all vehicle types, expressed
as indices relative to the average hour in a week (=100). These profiles are combined with
actual vehicle kilometres estimated for a single week, assuming total vehicle kilometres
spread evenly across 52 weeks in a year. The actual vehicle kilometre distribution for cars
was subtracted from the all vehicle types profile to produce distributions by time of day for
other vehicle types, as shown in Figure 5.
Figure 5. Distribution of traffic by time of day on all roads in Great Britain, 2010
Figure 5 shows that:

On an average weekday, there are two car traffic peaks between 7am and 8am in the
morning and between 4pm and 6pm in the afternoon. Friday has a slightly different
profile from other weekdays in that there is a lower peak in the morning, and the
traffic build ups steadily from around 10 am to the evening peak. At weekends, the
traffic peak occurs around 11am to 1pm, and there is no evening peak.

For other vehicle types (mainly goods/commercial vehicles), traffic remains at a
similar level between the morning and evening peaks on weekdays. The evening
peak occurs slightly earlier on Friday. At the weekend, traffic drops off significantly
below the average level, with the peak occurring in the late morning and gradually
falls off towards the evening.
Table 3 is a summary of the hourly variation in emissions from cars and other vehicles on
Mondays-Thursdays, Fridays and weekends.
Table 3: Fraction of Road Transport Emissions Emitted by Time of Day (2010)
Time of day
00:00-01:00
01:00-02:00
02:00-03:00
03:00-04:00
04:00-05:00
05:00-06:00
06:00-07:00
07:00-08:00
08:00-09:00
09:00-10:00
10:00-11:00
11:00-12:00
12:00-13:00
13:00-14:00
14:00-15:00
15:00-16:00
16:00-17:00
17:00-18:00
18:00-19:00
19:00-20:00
20:00-21:00
21:00-22:00
22:00-23:00
23:00-00:00
Total
Mon-Thu
0.0053
0.0030
0.0022
0.0024
0.0045
0.0132
0.0372
0.0740
0.0803
0.0607
0.0545
0.0547
0.0559
0.0573
0.0603
0.0680
0.0835
0.0899
0.0693
0.0444
0.0300
0.0226
0.0169
0.0099
1.000
Car
Fri
0.0056
0.0033
0.0025
0.0025
0.0040
0.0104
0.0287
0.0583
0.0643
0.0511
0.0535
0.0589
0.0635
0.0673
0.0712
0.0770
0.0825
0.0827
0.0681
0.0517
0.0364
0.0254
0.0184
0.0127
1.000
Weekend
0.0109
0.0066
0.0044
0.0037
0.0042
0.0072
0.0129
0.0218
0.0350
0.0532
0.0723
0.0836
0.0844
0.0797
0.0761
0.0756
0.0774
0.0747
0.0644
0.0508
0.0377
0.0279
0.0206
0.0148
1.000
Other Vehicles
Mon-Thu
Fri
Weekend
0.0093 0.0116
0.0194
0.0089 0.0109
0.0172
0.0091 0.0110
0.0159
0.0110 0.0127
0.0162
0.0168 0.0179
0.0185
0.0302 0.0301
0.0240
0.0559 0.0537
0.0342
0.0709 0.0702
0.0446
0.0669 0.0666
0.0512
0.0666 0.0659
0.0586
0.0647 0.0658
0.0655
0.0644 0.0685
0.0685
0.0644 0.0689
0.0674
0.0657 0.0703
0.0652
0.0679 0.0705
0.0624
0.0684 0.0671
0.0617
0.0654 0.0587
0.0606
0.0530 0.0465
0.0572
0.0414 0.0371
0.0506
0.0306 0.0292
0.0421
0.0232 0.0228
0.0337
0.0182 0.0179
0.0264
0.0148 0.0144
0.0211
0.0121 0.0118
0.0178
1.000
1.000
1.000
Figure 6 shows a series of charts illustrating how NO x, PM10 and NMVOCs emissions vary
by time of day on an average weekday (Mon-Thu), Friday and weekend, based on the traffic
profiles shown in Figure 5. Again, the overall hourly trend in emissions from traffic differ
slightly between pollutants due to the different contributions from cars and other vehicles.
Figure 6a. NOx emissions distribution on all roads by time of day (Mon-Thu, Friday
and weekend respectively).
Figure 6b. PM10 emissions distribution on all roads by time of day (Mon-Thu, Friday
and weekend respectively).
Figure 6c. NMVOCs emissions distribution on all roads by time of day (Mon-Thu,
Friday and weekend respectively).
Summary
The tables showing the relative trends in emissions by hour, day of week and month are
provided
in
an
Excel
spreadsheet
accompanying
this
note
(roadtransport_temporal_profile_2010_final.xlsx).
Recognising that some users may want to use in an air pollution model profiles that
represent all traffic emissions (rather than by vehicle type), the spreadsheet also provides
pollutant-specific profiles by hour for Monday-Thursday, Friday and weekends, by day of
week and by month for NOx, PM10, NMVOCs, CO, SO2 and NH3. These are weighted by the
contribution of cars and goods/other vehicles to the emissions inventory and therefore vary
by pollutant. They will also vary slightly by year as the relative contribution to traffic
emissions from the different vehicle types change over time.
As part of the current NAEI programme, these spreadsheet profiles will be updated annually
on the NAEI website.
A separate report covers the temporal profiles for other source sectors. Through a separate
Defra project, involving the development of CMAQ for national scale air pollution modelling,
an emission processor has also been developed which will use the profiles presented in this
work to calculate hourly emissions in a format suitable for air pollution models. The emission
processor will become available for use on all Defra air quality modelling projects.
The emission profiles here refer to hot exhaust emissions. They should also be valid for
other emissions which vary in proportion to traffic levels, such as non-exhaust emissions of
PM. Further research is required to develop profiles that represent the more complex
temporal variations in cold start and evaporative emissions from vehicles.
The temporal profiles covering all traffic emissions (all vehicle types) are also provided with
profiles for other source sectors in a spreadsheet ‘SNAPSector_temporal_profile_final.xlsx’
on this site.