
10Impact of Urban Trac on Fuel Consumption Leveraging IoT Data: Case Study of Riyadh City
3.1.3 Measuring Trac Congestion
Quantifying congestion can be done in many ways,
with numerous measures developed to assess dierent
performance criteria. These include speed, travel
time, delay, level of service, and congestion indices.
While congestion is often measured through various
means such as delay ratios and travel time indices
(TTI), our methodology focuses specifically on speed-
related indicators (Afrin and Yodo 2020). We will be
using the Speed Reduction Index (SRI) to measure
trac congestion, as it aligns with our objective of
understanding how variations in speed influence fuel
consumption. This is critical for quantifying the impact
of speed fluctuations on overall fuel eciency.
The SRI we are calculating is based on the dierence
between the average vehicle speed and the speed limit,
expressed as a ratio of the speed limit. The method
allows for a straightforward and intuitive measure of trac
congestion, reflecting to what extent trac has slowed
relative to ideal free-flow conditions. The SRI is calculated
as follows:
=
−
⎛
⎝
⎜⎞
⎠
⎟ (1)
This approach is aligned with the methodology presented
in Afrin and Yodo (2020) and Rao and Rao (2012) as
the formula captures the degree of speed reduction
under congested conditions. However, one adjustment
we are considering is to avoid multiplying the results
by 10, maintaining a scale from 0 to 1 instead of 0 to 10.
Additionally, we want to ensure the measure cannot be
negative, as we aim to exclude instances where vehicles
are traveling faster than the road’s speed limit, focusing
on free-flow trac conditions.
3.2 Estimating Fuel
Consumption
3.2.1 Incorporating Urban Driving
Conditions
Research has demonstrated that driving behaviors such
as accelerating, idling, speeding, and stop-and-go trac
patterns significantly influence a vehicle’s emissions.
Aggressive driving, for instance, typically leads to
greater fuel consumption and increased emissions
compared to steady, moderate driving (Wang et al.
2019). Frequent idling and constant variations in speed,
common in congested urban settings, also contribute to
greater emissions. Thus, understanding and modifying
driving behaviors can be a key strategy in reducing
vehicular carbon emissions (Wang et al. 2019). Similarly,
reducing trac congestion, which leads to stop-and-go
trac patterns, can also reduce vehicle emissions.
In this paper, we adopt a methodology similar to previous
studies that have examined the relationship between
average speed and both fuel consumption and emissions.
We utilize average speeds obtained from FCD data from
TomTom (2024a) to conduct our analysis. Previous studies
have developed specific parameters, primarily focusing
on vehicle speed and vehicle characteristics.
The relationship between fuel consumption and speed, as
highlighted in studies such as Barth and Boriboonsomsin
(2008), Sobrino, Monzón, and Hernández (2014), Song, Yu,
and Wu (2016), and Ricardo-AEA (2014), generally follows
a U-shaped curve. This means that fuel consumption
is higher at very low speeds, decreases as speed
approaches an optimal range, and then increases again
at higher speeds. This U-shape is influenced by factors
such as engine eciency, aerodynamic drag, and rolling
resistance, all of which vary based on vehicle type, as
these studies demonstrate. At lower speeds, engines are
less ecient, and more fuel is consumed to overcome
inertia and friction. As speed increases, engines enter
their optimal operating range, leading to reduced fuel
consumption per distance unit. However, at higher speeds,
aerodynamic drag becomes more dominant, demanding
more energy and causing fuel consumption to rise again.
The exact nature of this curve can vary depending on
vehicle type and engine size, as shown in the previous
studies. Barth and Boriboonsomsin (2008) created the
curves by using second-by-second vehicle velocity data,
integrating it with real-time trac data, segmenting it by
consistent levels of service, applying the Comprehensive
Modal Emissions Model to estimate CO2 emissions, and
fitting the results to a fourth-order polynomial.
Sobrino, Monzón, and Hernández (2014) collected
consumption factors from the COPERT IV database,
establishing fuel consumption curves based on speed
for each vehicle type, customizing these curves using
non-linear regression techniques, and adapting them
to specific case studies by considering the circulating
vehicle fleet composition and annual mileage
correction factors.
Song, Yu, and Wu (2016) collected real-world vehicle
activity data, developing speed-specific vehicle-specific