Questions
Transport routing is the process of finding the fastest or lowest-cost routes that connect places in a given transport network, and is a key step in transport accessibility analysis, fleet allocation and transport simulation and planning more broadly (Levinson et al. 2020). However, researchers and practitioners often face practical challenges when carrying out routing tasks due to the costs of licensed software, limited data availability, and the long computation times required to run multiple routing scenarios, particularly in large and complex multimodal transport networks.
While there are several open-source routing packages available for R, they either do not support public transport networks (Padgham 2019), or primarily focus on providing point-to-point routes rather than origin-destination travel time matrices (Morgan et al. 2019; Lovelace and Ellison 2019). Most routing algorithms find paths to all points in the network while finding a single route. Storing these paths rather than computing them for one origin-destination pair at a time is orders of magnitude more efficient. To our knowledge, no R package exists that supports these efficient many-to-many queries for public transport networks.
To fill this gap, this paper presents r5r, a new open-source R package for routing on multimodal transport networks based on the Rapid Realistic Routing on Real-world and Reimagined networks (R5) package. R5 is an open access routing engine written in Java and developed at Conveyal (Conway, Byrd, and van der Linden 2017; Conway, Byrd, and van Eggermond 2018) to provide an efficient backend for analytic applications, such as accessibility analysis. The r5r package provides an interface to run R5 locally from within R using seamless parallel computing. This tool can be used to address a variety of questions that require the efficient calculation of travel time matrices or the examination of multimodal transport routes.
Methods
The r5r package has low data requirements and is easily scalable, allowing fast computation of routes and travel times for either city or region-level analysis. It creates a routable transport network using street network data from OpenStreetMap (OSM) and optionally public transport data in the General Transit Feed Specification (GTFS) format.
The r5r package has 3 fundamental functions:
-
setup_r5(): builds a multimodal transport network used for routing in R5. This function automatically (1) downloads/updates a compiled R5 JAR file and stores it locally for future use; and (2) combines the OSM and GTFS datasets to build a routable network object. -
travel_time_matrix(): computes travel time estimates between one or multiple origin/destination pairs for a single departure time or for multiple departure times over atime_windowset by the user. This function uses an R5-specific extension to the RAPTOR routing algorithm which provides an efficient and systematic sampling of multiple simulated schedules when using frequency-based GTFS data (Conway, Byrd, and van der Linden 2017). -
detailed_itineraries(): computes detailed information on routes between one or multiple origin/destination pairs for a single departure time. The output includes detailed information on route alternatives such as the transport mode, waiting time, travel time and distance of each segment of the trip. This function uses an R5-specific extension[1] to the McRAPTOR (Delling, Pajor, and Werneck 2015) routing algorithm to find both optimal and slightly suboptimal paths.
Both routing functions are versatile so users can easily set customized inputs such as transport modes, departure dates and times, walking and cycling speeds, maximum trip duration, walking distances and number of public transport transfers. In the following section, we will focus on results obtained from travel_time_matrix().
Findings
After it is installed with the install.packages("r5r") command, the package can be attached (alongside other packages to reproduce this article), as in Code 1.
For this article, we used r5r version v0.3-2 and R5 version v6.0.1.
The package includes sample datasets for the cities of São Paulo and Porto Alegre (both in Brazil). Each dataset includes:
- An OSM network in
.pbfformat. - A public transport network in
GTFS.zipformat. - The spatial coordinates of points covering the area in
.csvformat, including information on the size of resident population and the number of schools in each location.
Building a routable transport network
To build a routable transport network with r5r and load it into memory, the user needs to call setup_r5 with the path to the directory where OSM and GTFS data are stored. In the examples herein, we use the provided Porto Alegre dataset (Code 2).
The function uses the .pbf and the GTFS.zip files in the directory pointed by data_path to create a multimodal transport network used for routing by R5. If multiple GTFS files are present, R5 will merge them into a single transport network. The resulting network.dat as well as some other files used by R5 are saved inside the supplied directory for later reuse.
Calculating a travel time matrix
The travel_time_matrix() function takes, as inputs, the spatial location of origins/destinations (either as a spatial sf POINT object, or as a data.frame containing the columns id, lon and lat) and a few travel parameters such as maximum trip duration, or walking distance. It outputs travel time estimates for each origin-destination pair at a set departure_datetime.
Since service levels can significantly vary across the day (Stępniak et al. 2019), r5r provides a time_window parameter that can help address the aggregation component of the modifiable temporal unit problem (MTUP) (Pereira 2019). When this parameter is set, R5 will compute travel times for trips at the specified departure time and every minute for time_window minutes after. The percentiles parameter allows the user to retrieve travel time estimates at different points of the distribution (by default the median). These percentiles reflect service variation over the time window, but do not reflect schedule deviation not represented in the GTFS, though tools exist to create GTFS which reflects schedule deviations (Wessel, Allen, and Farber 2017).
An example of the function’s usage is presented in Code 3. Computing this 1227x1227 travel time matrix with a 120-minute time window takes less than a minute on a Windows machine with a 1.9GHz Intel i7 and 16GB RAM.
Visualizing travel-time uncertainty
Figure 1 shows how the travel times to arrive at the central bus station from several origin points vary within the time window (5th, 25th, 50th, 75th, and 95th percentiles), reflecting that travel times are more uncertain when leaving from some places than others. While there is little to no uncertainty when departing from places that are very close (walking distance) to the central bus station, travel times from places farther away are more affected by departure time variations and service frequency levels.
Visualizing Isochrones
In our example, we can visualize the isochrone (area reachable within a certain amount of time) departing from the central bus station in Figure 2.
Creating accessibility metrics
Accessibility metrics measure the opportunities, such as jobs, a traveler could reach from a particular location (Levinson et al. 2020). One of the simplest forms is a cumulative-opportunities metric, which sums all of the opportunities accessible from each location in less than a cutoff time. Using the travel time matrix and information on the number of opportunities available at each location, we can calculate and map accessibility. In Figure 3 we compute the number of schools accessible by public transport in less than 20 minutes.
Acknowledgments
The R5 routing engine is developed at Conveyal with contributions from several developers. This work was supported by the Brazilian Institute for Applied Economic Research (Ipea).
The specific extension to McRAPTOR to do suboptimal path routing is not documented yet.