Core functions and classes

The package’s core functions are structured around 3 common types of spatial transport data:

• Origin-destination (OD) data, which report the number of people travelling between origin-destination pairs. This type of data is not explicitly spatial (OD datasets are usually represented as data frames) but represents movement over space between points in geographical space. An example is provided in the flow dataset.
• Line data, one dimensional linear features on the surface of the Earth. These are typically stored as a SpatialLinesDataFrame.
• Route data are special types of lines which have been allocated to the transport network. Routes typically result from the allocation of a straight ‘desire line’ allocated to the route network with a route_ function. Route network represent many overlapping routes. All are typically stored as SpatialLinesDataFrame.

For ease of use, functions focussed on each data type have been developed with names prefixed with od_, line_ and route_ respectively. A selection of these is presented in Table 1. Additional ‘core functions’ could be developed, such as those prefixed with rn_ (for working with route network data) and g_ functions for geographic operations such as buffer creation on lat/lon projected data (this function is currently named buff_geo). We plan to elicit feedback on such changes before implementing them.

With a tip of the hat to the concept of type stability (e.g. as implemented in ), we also plan to make the core functions of stplanr more type-stable in future releases. Core functions, which begin with the prefixes listed above, could follow ’s lead and return only objects with the same class as that of the input. However there are limitations to this approach: it will break existing functionality and mean that output objects have a larger size than necessary (line_bearing, for example, does not need to duplicate the spatial data contained in its input). Instead, we plan to continue to name functions around the type of input data they take, but are open minded about function input-output data class conventions, especially in the context of the new class system implemented in .

A class system has not been developed for each data type (this option is discussed in the final section). The most common data types used in stplanr are assumed to be data frames and spatial datasets.

Transport datasets are very diverse. There are therefore many other functions which have more ad-hock names. Rather attempt a systematic description of each of stplanr’s functions (which can be gleaned from the online manual) it is more illuminating to see how they work together, as part of a transport planning workflow. As with most workflows, this begins with data access and ends with visualisation.

Accessing and processing transport data

Gaining access to data is often the first stage in transport research. This is often a long and protracted process which is thankfully becoming easier thanks to the ‘open data’ movement and packages such as tigris for making data access from within R easier .

stplanr provides a variety of different functions that facilitate importing common data formats used for transport analysis into R. Although transport analysis generally requires some transport-specific datasets, it also typically relies heavily on common sources of data including census data. This being the case, stplanr also includes functions that may be useful to those not involved in transport research. This includes the read_table_builder function for importing data from the Australian Bureau of Statistics (ABS) and the UK’s Stats19 road traffic casualty dataset. A brief example of the latter is demonstrated below, which begins with downloading the data (warning this downloads ~100 MB of data):

dl_stats19() # download and extract stats19 road traffic casualty data

#> [1] "Data saved at: /tmp/RtmpppF3E2/Accidents0514.csv"
#> [2] "Data saved at: /tmp/RtmpppF3E2/Casualties0514.csv"
#> [3] "Data saved at: /tmp/RtmpppF3E2/Vehicles0514.csv"  

Once the data has been saved in the default directory, determined by tempdir(), it can be read-in and cleaned with the read_stats19_ functions (note these call format_stats19_ functions internally to clean the datasets and add correct labels to the variables):

ac <- read_stats19_ac()
ca <- read_stats19_ca()
ve <- read_stats19_ve()

The resulting datasets (representing accident, casualty and vehicle level data, respectively) can be merged and made geographic, as illustrated below:

library(dplyr)
ca_ac <- inner_join(ca, ac)
ca_cycle <- ca_ac %>%
  filter(Casualty_Severity == "Fatal" & !is.na(Latitude)) %>%
  select(Age = Age_of_Casualty, Mode = Casualty_Type, Longitude, Latitude)
ca_sp <- SpatialPointsDataFrame(coords = ca_cycle[3:4], data = ca_cycle[1:2])

Now that this casualty data has been cleaned, subsetted (to only include serious cycle crashes) and converted into a spatial class system, we can analyse them using geographical datasets of the type commonly used by stplanr. The following code, for example, geographically subsets the dataset to include only crashes that occured within the bounding box of a route network dataset provided by stplanr (from version 0.1.7 and beyond) using the function bb2poly, which converts a spatial dataset into a box, represented as a rectangular SpatialPolygonsDataFrame:

data("route_network") # devtools::install_github("ropensci/splanr")version 0.1.7
proj4string(ca_sp) <- proj4string(route_network)
bb <- bb2poly(route_network)
proj4string(bb) <- proj4string(route_network)
ca_local <- ca_sp[bb, ]

The above code chunk shows the importance of understanding geographical data when working with transport data. It is only by converting the casualty data into a spatial data class, and adding a coordinate reference system (CRS), that transport planners and researchers can link this important dataset back to the route network. We can now perform GIS operations on the results. The next code chunk, for example, finds all the fatalities that took place within 100 m of the route network, using the function buff_geo:

rnet_buff_100 <- geo_buffer(route_network, width = 100)
ca_buff <- ca_local[rnet_buff_100, ]

These can be visualised using base R graphics, extended by , as illustrated in Figure . This provides a good start for analysis but for publication-quality plots and interactive plots, designed for public engagement, we recommend using dedicated visualisation packages that work with spatial data such as .

plot(bb, lty = 4)
plot(rnet_buff_100, col = "grey", add = TRUE)
points(ca_local, pch = 4)
points(ca_buff, cex = 3)

Creating geographic desire lines

Perhaps the most common type of aggregate-level transport information is origin-destination (‘OD’) data. This can be presented either as a matrix or (more commonly) a long table of OD pairs. An example of this type of raw data is provided below (see ?flow to see how this dataset was created).

data("flow", package = "stplanr")
head(flow[c(1:3, 12)])

Although the flow data displayed above describes movement over geographical space, it contains no explicitly geographical information. Instead, the coordinates of the origins and destinations are linked to a separate geographical dataset which also must be loaded to analyse the flows. This is a common problem solved by the function od2line. The geographical data is a set of points representing centroids of the origin and destinations, saved as a SpatialPointsDataFrame. Geographical data in R is best represented as such Spatial* objects, which use the S4 object engine. This explains the close integration of stplanr with R’s spatial packages, especially sp, which defines the S4 spatial object system.

data("cents", package = "stplanr")
as.data.frame(cents[1:3, -c(3, 4)])

We use od2line to combine flow and cents, to join the former to the latter. We will visualise the l object created below in the next section.

l <- od2line(flow = flow, zones = cents)

The data is now in a form that is much easier to analyse. We can plot the data with the command plot(l), which was not possible before. Because the SpatialLinesDataFrame object also contains data per line, it also helps with visualisation of the flows, as illustrated in Figure .

Allocating flows to the transport network

A common problem faced by transport researchers is network allocation: converting the ‘as the crow flies’ lines illustrated in the figure above into routes. These are the complex, winding paths that people and animals make to avoid obstacles such as buildings and to make the journey faster and more efficient (e.g. by following the route network).

This is difficult (and was until recently near impossible using free software) because of the size and complexity of transport networks, the complexity of realistic routing algorithms and need for context-specificity in the routing engine. Inexperienced cyclists, for example, would take a very different route than a heavy goods vehicle. stplanr tackles this issue by using 3rd party APIs to provide route-allocation.

Route allocation is undertaken by functions such as and . These allocate a single OD pair, represented as a text string to be ‘geo-coded’, a pair of of coordinates, or two SpatialPoints objects, representing origins and destinations. This is illustrated below with route_cyclestreet, which uses the CycleStreets.net API, a routing service “by cyclists for cyclists” that offers a range route strategies (primarily ‘fastest’, ‘quietest’ and ‘balanced’) that are based on a detailed analysis of cyclist wayfinding:²

route_bl <- route_cyclestreets(from = "Bradford", to = "Leeds")
route_c1_c2 <- route_cyclestreets(cents[1, ], cents[2, ])

The raw output from routing APIs is usually provided as a JSON or GeoJSON text string. By default, route_cyclestreet saves a number of key variables (including length, time, hilliness and busyness variables generated by CycleStreets.net) from the attribute data provided by the API. If the user wants to save the raw output, the save_raw argument can be used:

route_bl_raw <- route_cyclestreets(from = "Bradford", to = "Leeds", save_raw = TRUE)

Additional arguments taken by the route_ functions depend on the routing function in question. By changing the plan argument of route_cyclestreet to fastest, quietest or balanced, for example, routes favouring speed, quietness or a balance between speed and quietness will be saved, respectively.

To automate the creation of route-allocated lines over many desire lines, the line2route function loops over each line, wrapping any route_ function as an input. The output is a SpatialLinesDataFrame with the same number of dimensions as the input dataset (see the right panel in Figure ).

routes_fast <- line2route(l = l, route_fun = route_cyclestreet)

The result of this ‘batch routing’ exercise is illustrated in Figure . The red lines in the left hand panel are very different from the hypothetical straight ‘desire lines’ often used in transport research, highlighting the importance of this route-allocation functionality.

plot(route_network, lwd = 0)
plot(l, lwd = l$All / 10, add = TRUE)
lines(routes_fast, col = "red")
routes_fast$All <- l$All
rnet <- overline(routes_fast, "All", fun = sum)
rnet$flow <- rnet$All / mean(rnet$All) * 3
plot(rnet, lwd = rnet$flow / mean(rnet$flow))

To estimate the amount of capacity needed at each segment on the transport network, the overline function demonstrated above, is used to divide line geometries into unique segments and aggregate the overlapping values. The results, illustrated in the right-hand panel of Figure , can be used to estimate where there is most need to improve the transport network, for example informing the decision of where to build new bicycle paths.

Limitations with the route_cyclestreet routing API include its specificity, to one mode (cycling) and a single region (the UK and part of Europe). To overcome these limitations, additional routing APIs were added with the functions route_graphhopper, route_transportapi_public and viaroute. These interface to Graphhopper, TransportAPI and the Open Source Routing Machine (OSRM) routing services, respectively. The great advantage of OSRM is that it allows you to run your own routing services on a local server, greatly increasing the rate of route generation.

A short example of finding the route by car and bike between New York and Oaxaca demonstrates how route_graphhopper can collect geographical and other data on routes by various modes, anywhere in the world. The output, shown in Table , shows that the function also saves time, distance and (for bike trips) vertical distance climbed for the trips.

ny2oaxaca1 <- route_graphhopper("New York", "Oaxaca", vehicle = "bike")
ny2oaxaca2 <- route_graphhopper("New York", "Oaxaca", vehicle = "car")
rbind(ny2oaxaca1@data, ny2oaxaca2@data)

Modelling travel catchment areas

Accessibility to transport services is a particularly important topic when considering public transport or active travel because of the frequent steep reduction in use as distances to access services (or infrastructure) increase. As a result, the planning for transport services and infrastructure frequently focuses on several measures of accessibility including distance, but also travel times and frequencies and weighted by population. The functions in stplanr are intended to provide a method of estimating these accessibility measures as well as calculating the population that can access specific services (i.e., estimating the catchment area).

Catchment areas in particular are a widely used measure of accessibility that attempts to both quantify the likely target group for a particular service, and visualise the geographic area that is covered by the service. For instance, passengers are often said to be willing to walk up to 400 metres to a bus stop, or 800 metres to a railway station . Although these distances may appear relatively arbitrary and have been found to underestimate the true catchment area of bus stops and railway stations they nonetheless represent a good, albeit somewhat conservative, starting point from which catchment areas can be determined.

In many cases, catchment areas are calculated on the basis of straight-line (or “as the crow flies”) distances. This is a simplistic, but relatively appealing approach because it requires little additional data and is straight-forward to understand. stplanr provides functionality that calculates catchment areas using straight-line distances with the calc_catchment function. This function takes a SpatialPolygonsDataFrame that contains the population (or other) data, typically from a census, and a Spatial* layer that contains the geometry of the transport facility. These two layers are overlayed to calculate statistics for the desired catchments including proportioning polygons to account for the proportion located within the catchment area.

To illustrate how catchment areas can be calculated, stplanr contains some sample datasets stored in ESRI Shapefile format (a commonly used format for distributing GIS layers) that can together be used to calculate sample catchment areas. One of these datasets (smallsa1) contains population data for Statistical Area 1 (SA1) zones in Sydney, Australia. The second contains hypothetical cycleways aligned to streets in Sydney. The code below unzips the datasets and reads in the shapefiles.

data_dir <- system.file("extdata", package = "stplanr")
unzip(file.path(data_dir, "smallsa1.zip"))
unzip(file.path(data_dir, "testcycleway.zip"))
sa1income <- as(sf::read_sf("smallsa1.shp"), "Spatial")
testcycleway <- as(sf::read_sf("testcycleway.shp"), "Spatial")
# Remove unzipped files
file.remove(list.files(pattern = "^(smallsa1|testcycleway).*"))

Calculating the catchment area is straightforward and in addition to specifying the required datasets, only a vector containing column names to calculate statistics and a distance is required. Since proportioning the areas assumes projected data, unprojected data are automatically projected to either a common projection (if one is already projected) or a specified projection. It should be emphasised that the choice of projection is important and has an effect on the results meaning setting a local projection is recommended to achieve the most accurate results.

remotes::install_github("ropensci/stplanr")
catch800m <- calc_catchment(
  polygonlayer = sa1income,
  targetlayer = testcycleway,
  calccols = c("Total"),
  distance = 800,
  projection = "austalbers",
  dissolve = TRUE
)

By looking at the data.frame associated with the SpatialPolygonsDataFrame that is returned from the calc_catchment function, the total population within the catchment area can be seen to be nearly 40,000 people. The catchment area can also be plotted as with any other Spatial* object using the plot function using the code below with the result shown in Figure .

plot(sa1income, col = "light grey")
plot(catch800m, col = rgb(1, 0, 0, 0.5), add = TRUE)
plot(testcycleway, col = "green", add = TRUE)

This simplistic catchment area is useful when the straight-line distance is a reasonable approximation of the route taken to walk (or cycle) to a transport facility. However, this is often not the case. The catchment area in Figure initially appears reasonable but the red-shaded catchment area includes an area that requires travelling around a bay to access from the (green-coloured) cycleway. To allow for more realistic catchment areas for most situations, stplanr provides the calc_network_catchment function that uses the same principle as calc_catchment but also takes into account the transport network.

To use calc_network_catchment, a transport network needs to be prepared that can be used in conjunction with the previous datasets. Preparation of the dataset involves using the SpatialLinesNetwork function to create a network from a SpatialLinesDataFrame. This function combines a SpatialLinesDataFrame with a graph network (using the package) to provide basic routing functionality. The network is used to calculate the shortest actual paths within the specific catchment distance. This process involves the following code:

unzip(file.path(data_dir, "sydroads.zip"))
sydroads <- as(sf::read_sf(".", "roads"), "Spatial")
file.remove(list.files(pattern = "^(roads).*"))
sydnetwork <- SpatialLinesNetwork(sydroads)

The network catchment is then calculated using a similar method as with calc_catchment but with a few minor changes. Specifically these are including the SpatialLinesNetwork, and using the maximpedance parameter to define the distance, with distance being the additional distance from the network. In contrast to the distance parameter that is based on the straight-line distance in both the calc_catchment and calc_network_catchment functions, the maximpedance parameter is the maximum value in the units of the network’s weight attribute. In practice this is generally distance in metres but can also be travel times, risk or other measures.

netcatch800m <- calc_network_catchment(
  sln = sydnetwork,
  polygonlayer = sa1income,
  targetlayer = testcycleway,
  calccols = c("Total"),
  maximpedance = 800,
  distance = 100,
  projection = "austalbers"
)

Once calculated, the network catchment area can be used just as the straight-line network catchment. This includes extracting the catchment population of 128,000 and plotting the original catchment area together with the original area with the results shown in Figure :

plot(sa1income, col = "light grey")
plot(catch800m, col = rgb(1, 0, 0, 0.5), add = TRUE)
plot(netcatch800m, col = rgb(0, 0, 1, 0.5), add = TRUE)
plot(testcycleway, col = "green", add = TRUE)

Modelling mode choice

Route-allocated lines allow estimation of route distance and cirquity (route distance divided by Euclidean distance). These variables can help model the rate of flow between origins and destination, as illustrated in the left-hand panel of Figure . The code below demonstrates how objects generated by stplanr can be used to undertake such analysis, with the line_length function used to find the distance, in meters, of lat/lon data.

l$d_euclidean <- line_length(l)
l$d_rf <- routes_fast@data$length
plot(l$d_euclidean, l$d_rf,
  xlab = "Euclidean distance", ylab = "Route distance")
abline(a = 0, b = 1)
abline(a = 0, b = 1.2, col = "green")
abline(a = 0, b = 1.5, col = "red")

The left hand panel of Figure shows the expected strong correlation between Euclidean (\(d_E\)) and fastest route (\(d_{Rf}\)) distance. However, some OD pairs have a proportionally higher route distance than others, as illustrated by distance from the black line in the above plot: this represents : the ratio of network distance to Euclidean distance :

\[ Q = \frac{d_{Rf}}{d_E} \]

An extension to the concept of cirquity is the ‘quietness diversion factor’ (\(QDF\)) of a desire line , the ratio of the route distance of a quiet route option (\(d_{Rq}\)) to that of the fastest:

\[ QDF = \frac{d_{Rq}}{d_{Rf}} \]

Thanks to the ‘quietest’ route option provided by route_cyclestreet, we can estimate average values for both metrics as follows:

routes_slow <- line2route(l, route_cyclestreet, plan = "quietest")

l$d_rq <- routes_slow$length # quietest route distance
Q <- mean(l$d_rf / l$d_euclidean, na.rm = TRUE)
QDF <- mean(l$d_rq / l$d_rf, na.rm = TRUE)
Q
QDF

The results show that cycle paths are not particularly direct in the study region by international standards . This is hardly surprisingly given the small size of the sample and the short distances covered: \(Q\) tends to decrease at a decaying rate with distance. What is surprising is that \(QDF\) is close to unity, which could imply that the quiet routes are constructed along direct, and therefore sensible routes. We should caution against such assumptions, however: It is a small sample of desire lines and, when time is explored, we find that the ‘quietness diversion factor with respect to time’ (\(QDF_t\)) is slightly larger:

(QDFt <- mean(routes_slow$time / routes_fast$time, na.rm = TRUE))

Models of travel behaviour

There are many ways of estimating flows between origins and destinations, including spatial interaction models, the four-stage transport model and gravity models (‘distance decay’). stplanr aims eventually to facilitate creation of many types of flow model.

At present there are no functions for modelling distance decay, but this is something we would like to add in future versions of stplanr. Distance decay is an especially important concept for sustainable transport planning due to physical limitations on the ability of people to walk and cycle large distances .

We can explore the relationship between distance and the proportion of trips made by walking, using the same object l generated by stplanr.

l$pwalk <- l$On.foot / l$All
plot(l$d_euclidean, l$pwalk,
  cex = l$All / 50,
  xlab = "Euclidean distance (m)", ylab = "Proportion of trips by foot"
)

Based on the right-hand panel in Figure , there is a clear negative relationship between distance of trips and the proportion of those trips made by walking. This is unsurprising: beyond a certain distance (around 1.5km according the the data presented in the figure above) walking is usually seen as too slow and other modes are considered. According to the academic literature, this ‘distance decay’ is non-linear and there have been a number of functions proposed to fit to distance decay curves . From the range of options we test below just two forms. We will compare the ability of linear and log-square-root functions to fit the data contained in l for walking.

lm1 <- lm(pwalk ~ d_euclidean, data = l@data, weights = All)
lm2 <- lm(pwalk ~ d_rf, data = l@data, weights = All)
lm3 <- glm(pwalk ~ d_rf + I(d_rf^0.5),
  data = l@data, weights = All, family = quasipoisson(link = "log")
)

The results of these regression models can be seen using summary(). Surprisingly, Euclidean distance was a better predictor of walking than route distance, but no strong conclusions can be drawn from this finding, with such a small sample of desire lines (n = 42). The results are purely illustrative, of the kind of the possibilities created by using stplanr in conjuction with R’s modelling capabilities (see Figure ).

plot(l$d_euclidean, l$pwalk,
  cex = l$All / 50,
  xlab = "Euclidean distance (m)", ylab = "Proportion of trips by foot"
)
l2 <- data.frame(d_euclidean = 1:5000, d_rf = 1:5000)
lm1p <- predict(lm1, l2)
lm2p <- predict(lm2, l2)
lm3p <- predict(lm3, l2)
lines(l2$d_euclidean, lm1p)
lines(l2$d_euclidean, exp(lm2p), col = "green")
lines(l2$d_euclidean, exp(lm3p), col = "red")

Visualisation

Visualisation is an important aspect of any transport study, as it enables researchers to communicate their findings to other researchers, policy-makers and, ultimately, the public. It may therefore come as a surprise that stplanr contains no functions for visualisation. Instead, users are encouraged to make use of existing spatial visualisation tools in R, such as tmap, leaflet and ggmap .

Furthermore, with the development of online application frameworks such as shiny, it is now easier than ever to make the results of transport analysis and modelling projects available to the public. An example is the online interface of the Propensity to Cycle Tool (PCT). The results of the project, generated using stplanr, are presented at zone, desire line and Route Network levels . There is great potential to expand on the principle of publicly accessible transport planning tools via ‘web apps’, perhaps through new R packages dedicated to visualising transport data.