This section presents the results we obtained from our study, divided in three main parts. First, we describe the predictions obtained from the simulation model, then those we get for air quality, and finally the exposure to PM2.5 computed by compounding air quality measurements with the simulated travel times, according to equation 4.
Simulation Results
Figure 3 shows a map of Bogota urban area divided by UPZ, with the currently planned QC route highlighted ´ in orange. The route is divided into 119 segments, each segment modeling a straight portion of the bikeway. As a notation, we number segments consecutively, with segment 1 being the southernmost one and segment 119 the northernmost one. Also, we label with North and South the cyclists’ trips that go towards that direction. Multiple simulation runs were aggregated to compute averages values of the measures of interest and confidence intervals with 95% confidence level.
The simulated average times cyclist spend in segments of the route (the EDi factors of equation 4) are shown in figure 4, in hours, for trips going north and south, and for the three time ranges considered. This average time depends on the number of cyclists that are sharing the segment, their speeds and the length of the segment.
Table 1: Average time (in minutes) across all bikeway segments, per time range.
First of all, we consistently observe a peak in the first, southernmost segments. This is due to the very high number of trips originating in the area at the very end of the QC, most of which go further south and therefore fall out of the scope of our study. During the morning rush hour (time-range 1, left chart), more people are expected to head towards the north of the city (Secretar´ıa de Movilidad Alcald´ıa Mayor de Bogota TPD Ingenier ´ ´ıa 2016). As our simulation results indicate, segments 80 to 115 show the highest average times. In the second time-range (middle chart), the peaks of the time spent by the cyclist are found in the southern segments. Also, time variability across segments is reduced. The simulation results for the last time-rage (right chart) shows that in the central segments people can spend as much as twice the time spent in other segments of the route. This could be because most of the trips in this time range originate in the south and north of the city, and end in the opposite side of the city, with people accumulating at the middle of the QC route.
Consistently across all time-ranges, people heading south spend more time in each segment than people going the other direction. This is because traffic going south is of higher intensity, and cyclists would have to slow down and stay longer on a segment. The average sojourn time across all segments, reported in table 1 for each time trip direction and time range together with the half-width of the confidence interval, statistically confirms the existence of this difference.
Air Quality Results
From the Kriging interpolated air quality data, we estimate the average PM2.5 concentration for each segment in the QC (the Ci factors in equation 4), which we show in figure 5. The morning and night time ranges are rush hours, and have a higher flow of vehicles on the streets. Accordingly, figure 5 shows that for those time ranges higher concentrations of PM2.5, than those of the noon time-range. For some segments in the southern part of the QC (left part of the curves), the concentration in time range 1 is 2.5 times the concentration in time range 2.
In time range 1, concentrations of PM2.5 are at their maximum levels. We observe that the highest concentrations are estimated for the southernmost segments of the QC, where the largest expected sojourn times are estimated (see figure 4). Even though the intersection between the set of highly polluted and the set of congested segments is of small cardinality, it should raise concerns about the health implications of the high exposure. On the contrary, the comparatively lower levels of PM2.5 on the northern QC segments would compensate in the exposure assessment the long sojourn times of cyclists in time range 1 (see figure 4.a).
In the second time-range, the concentrations of PM2.5 are at their minimum, as the traffic intensity in the city is much lower than in the morning. This is explained based on the normal working shift of the people. Hence, based only on the concentration variable, it could be concluded that the second time-range would the best moment to use the QC.
The last range reports smaller average values than the first one. Even if in the time range 3 a number of trip approximately equal to the one of time range 1 takes place, two distinct factors contribute to determine lower averages of PM2.5 concentration: the first one is the largest spread of the trip starting times, and the second one is the more efficient dispersion of pollutants in the atmosphere later in the day.
Exposure Results
The exposure, calculated according to equation 4, depends on the cyclists characteristics and on the route. Therefore, result can only be computed with reference to specific trips of specific users. Then, to evaluate exposure, we generated random profiles of bikeway users who would move along the QC path. The random generation of profiles is based on the information of the EM survey.
The segment-wise estimated exposure of the random generated mix of cyclists along the route is shown in figure 6). As expected, the behavior of the exposition along the route is very similar to the average time in the segment. However, significant differences exist among profiles, as women and young people will have higher exposure in almost every segment along the route, due to their higher ventilation rates (V R factor in equation 4). To provide a more precise characterization of the differences in the exposure determined by the cyclist profile, we report in table 2 the estimated AD for a set of sampled profiles, assuming that the trip goes along the whole QC.
The values reported in table 2 help gaining an understanding of the magnitude of the cyclists’ exposure along the route. Studies in the literature show that 24-hour exposure in small cities (Lee et al. 2017) can be around 4.6 µgm−3d −1 . With the levels of pollution in Bogota, a couple of hours along the bikeway ´ would results in a similar amount of inhaled PM2.5.
Table 2: Average exposure PM2.5 for cyclists traveling QC in different time ranges and direction.
Conclusions
In this study, we describe the combined use of a traffic simulation model and air quality data to generate predictions about the exposure of cyclists to PM2.5 along Quinto Centenario, a 25-km long bikeway that will be built in Bogota.
The purpose of the simulation model is to provide estimates for the travel times of users, broken down into the time spent in the distinct segments that compose the modeled bikeway. An essential part of the our work focuses on the parametrization of the simulation model, to ensure the demand of bike trips, their characteristics in terms of origin/destination and speed are indeed capturing the real behavior of bikers in the city. Official data from a comprehensive survey collected by local authorities is used to determine the influence zone of the planned bikeway route, model the trip arrival process, estimate an ODM and speed of trips.
The information about air quality along the bikeway is obtained by the spatial interpolation of the official city data collected by a network of monitoring stations. By combining the spatial distribution of PM2.5 concentrations with the average time cyclists would spent along the bikeway, we can obtain estimates for the cumulative exposure of bikeway users according to the suggested EPA metrics for inhalation along routes.
Knowing a persons gender and age allows calculating exposure in terms of the predicted average amount of PM2.5 that a cyclist would inhale in a bike trip along Quinto Centenario. This information is valuable for both people working on the design of the route and for its users. The first ones can use it to compare the impact on health of different routes options, while the latter ones can make an informed decision about the correct physical barrier they can use to protect themselves from the effects of long-term exposure to pollutants. The preliminary results of this work have been presented to the mobility authority of Bogota.´ We are currently working on the development of an improved simulation model that allows considering a better characterization of the exposure for high altitude cities, as well as on the evaluation of the overall cost-benefit of performing physical activity in polluted environments.
Author biographies
DANIELA AZUMENDI GONGORA ´ is a graduated teacher assistant at Universidad de los Andes, where she supports the Discrete Event Simulation course. She completed a major in industrial engineering and a double program in mechanical engineering in 2018, and is currently a student of the industrial engineering master program at the same university. She is interested in the the applications of operations research to environment protection and social welfare. Her email address is d.azumendi10@uniandes.edu.co.
JUAN JOSE D´ ´IAZ BAQUERO is a magister in industrial engineering with double program degree in software engineering and industrial engineering from Universidad de los Andes, with experience in IT consulting, data analytics and visual analytics. Passionate about operations research, mathematical modeling, simulation, programming, climate change and public health. His email address is jj.diaz1067@uniandes.edu.co.
JUAN FELIPE FRANCO got a chemical engineering degree from Universidad Nacional de Colombia, a master in engineering from Universidad de los Andes, and is currently a Ph.D. student in the engineering program at that same university. He has experience as a teacher, researcher and consultant in topics related to air pollution control, reduction of greenhouse gas emission, urban sustainability management and public policy definition. His email address is jffranco@uniandes.edu.co.
IVAN MURA received his first degree in Computer Science and a Ph.D. in computer science engineering from the University of Pisa, Italy, and a master of science in information technology project management from George Washington University School of Business. He is currently an associate professor at the Department of industrial engineering, Universidad de los Andes. His research interests include the mathematical modeling of artificial and living systems, with continuous-deterministic and discrete-stochastic state-based techniques. His e-mail address is i.mura@uniandes.edu.co.
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