The original paper link is here.
In this paper, the authors evaluate the impact of horizontal highway geometric design features on CO2 emissions with VT-Micro, a microscopic emission estimation model. The authors believe that too little research thus far has focused on the effects of horizontal alignment on GHG emissions, compared with the number of studies that evaluate the influence of vertical alignment. The authors study a database of more than 16,000 veh-km collected in 2008 on 11 two-lane rural road sections which were divided into 29 homogeneous road segments according to their Curvature Change Rate (CCR). To estimate CO2 emissions, instantaneous speed and acceleration are used as input variables for the VT-micro model. The paper provides geometric analysis, operational analysis, and combined analysis. As for geometric analysis, the study focuses on CCR, because of the close relationship between this geometric variable and CO2 emissions. Three different speeds are studied in the operational analysis: 50th percentile speed, 85th percentile speed and average speed. Geometric and operational variables are analyzed together in the combined analysis.
Based the research, CO2 emission rates increase with CCR; and lower speeds and higher speed dispersion can lead to higher CO2 emissions [1].
This paper argues for the importance of road horizontal design, especially CCR, in terms of reducing CO2 emissions. In my opinion, this approach could compliment the better-known current approaches, which include promoting electric cars, using more fuel-efficient vehicles, charging green-tax, etc.
When analysing traffic-related emissions, traffic simulation and emission models can be used [2]. However, instead of using traffic simulation models to obtain velocity and acceleration, the authors collected field data to estimate CO2 emissions, which is because, in my opinion, they want to prove, through a more accurate operational analysis with actual data, that geometric design plays an important role in changing driver operation such as speed and accelerations.
In the end of the paper, the authors indicate that their study may contribute to the development of advanced navigation systems which could estimate emissions as a function of road geometric design instead of or complementing other variables such as traffic volume density, and average speed [1]. I agree with this idea partially. Geometric factors could be used as inputs in traffic simulation models to obtain or calibrate speed and accelerations, before applying emission models. However, geometric design may not be able to replace other variables the authors mentioned. The reason why geometric factors would influence CO2 emissions is those factors would impact speed and accelerations which are directly associated with emissions. If geometry is the only factor impacting speed and accelerations, then emissions may be a function of geometric factors. But other variables, such as locations and road grades, could also impact traffic speed and accelerations.
In my opinion, considering geometric features, such as slopes and curves, is beneficial to addressing CO2 emission issues when designing roads. When designing roads, besides following road and traffic engineering design standards, designers can also try to apply lower slopes and CCR, because lower CCR contributes to lower CO2 emissions [1], and larger amounts of CO2 are produced when driving on roads with higher slopes [3]. In further studies, the margin benefit and margin cost of geometric features may be studied to find out the best geometry design standards for reducing traffic CO2 emissions.
References
[1] Llopis-Castelló, D., Pérez-Zuriaga, A. M., Camacho-Torregrosa, F. J. and García, A., “Impact of horizontal geometric design of two-lane rural roads on vehicle CO2 emissions,” Transportation Research Part D, pp. 46-57, 2018. [2] Luin, B. and Petelin, S., “Impact of road geometry on vehicle energy consumption,” transport problems, vol. 12, no. 2, 2017. [3] Chang, C. and Lin, T., “Estimation of Carbon Dioxide Emissions Generated by Building and Traffic in Taichung City,” Sustainablility, pp. 1-18, 2018.
