multinet_traveltimes: Calculate travel times from a single origin to all...

In triply-at/gtfsmulti: Creating, storing and analyzing multi-modal transportation networks

Description

This function calculates travel times in a multi-modal network from a specified location anywhere within the grid of a GTFS-Multi feed to all grid points of that grid. The travel times are the shortest multi-modal travel times within the bounds of acceptable departure times. Travel times may also be single-modal in case it is faster to travel directly from the origin to the destination, i.e. without using transit at all.

Usage

multinet_traveltimes(
  x,
  from,
  start_time_limits,
  access_modes = NULL,
  direct_modes = NULL,
  egress_modes = NULL,
  transfer_modes = NULL,
  access_limit = Inf,
  direct_limit = Inf,
  egress_limit = Inf,
  transfer_limit = Inf,
  minimise_transfers = FALSE,
  include_grid_access = TRUE,
  iterate = FALSE,
  quiet = FALSE,
  access_frequency = 60,
  grid_access_speed = 5/3.6,
  router_timeout = 60 * 60,
  day = NULL,
  date = NULL
)

Arguments

x	An object of class multinet, which is a list of multiple data.table objects corresponding to the different dataset files of a GTFS-Multi feed.
from	Origin of the routes. May be the index of a grid point. In that case it should correspond to a value in the stop_id column of the grid table in x. May also be any location inside the extent of the grid. Such a location may be given as a single POINT geometry inside an object of either class sf or sfc. May also be given as a numeric vector containing the two coordinate values. In any case, coordinates are expected to be expressed as longitude and latitude values with WGS84 as geodetic datum.
start_time_limits	A vector of two integer values denoting the start and end of the time interval in which trips may be started. Values should be given in seconds since midnight.
access_modes	Vector of transport modes to consider for access trips, i.e. trips from grid points towards transit stop locations. Each of these transport modes should have their access travel time for each access trip specified in the access table of x, in a column named transfer_time_<mode>. If NULL, all modes that have such a column in the access table are considered. If NA, access trips are not included at all. Defaults to NULL.
direct_modes	Vector of transport modes to consider for direct trips, i.e. trips between grid points. Each of these transport modes should have their direct travel time for each direct trip specified in the direct table of x, in a column named transfer_time_<mode>. If NULL, all modes that have such a column in the direct table are considered. If NA, direct trips are not included at all. Defaults to NULL.
egress_modes	Vector of transport modes to consider for egress trips, i.e. trips from transit stop locations towards grid points. Each of these transport modes should have their egress travel time for each egress trip specified in the egress table of x, in a column named transfer_time_<mode>. If NULL, all modes that have such a column in the egress table are considered. If NA, egress trips are not included at all. Defaults to NULL.
transfer_modes	Vector of transport modes to consider for true transit transfer trips, i.e. trips between transit stop locations. Each of these transport modes should have their transfer travel time for each transfer trip specified in the transfer table of x, in a column named transfer_time_mode. If NULL, all modes that have such a column in the transfer table are considered. If NA, transfer trips are not included at all. Defaults to NULL.
access_limit	Maximum time in seconds for access trips. Any access trip in the access table of x with a travel time higher than this limit will not be considered in the travel time calculation. Defaults to Inf, meaning that all specified access trips will be considered.
direct_limit	Maximum time in seconds for direct trips. Any direct trip in the direct table of x with a travel time higher than this limit will not be considered in the travel time calculation. Defaults to Inf, meaning that all specified direct trips will be considered.
egress_limit	Maximum time in seconds for egress trips. Any egress trip in the egress table of x with a travel time higher than this limit will not be considered in the travel time calculation. Defaults to Inf, meaning that all specified egress trips will be considered.
transfer_limit	Maximum time in seconds for transfer trips. Any transfer trip in the transfer table of x with a travel time higher than this limit will not be considered in the travel time calculation. Defaults to Inf, meaning that all specified transfer trips will be considered.
minimise_transfers	Boolean. If TRUE, travel times are calculated for routes with minimal transfer connections between the origin and each destination, even if those are slower than travel times of alternative routes with more transfers. Defaults to FALSE.
include_grid_access	Boolean. If TRUE, the straight-line distance between the origin location and its nearest grid point is assumed to be travelled at the speed given by the grid_access_speed parameter. The resulting travel time is added to the calculated travel times originating from the nearest grid point. If FALSE, this "pre-leg" of the route is ignored. Defaults to TRUE.
iterate	Boolean. If TRUE, a separate route search is performed for every possible departure time of an access trip within the time interval. For each of these departure times, the shortest travel times to all destinations are returned in a separate column, with the restriction that trips need to start exactly at this time, instead of at anytime within a time interval. This is similar to the R5 approach for multi-modal routing. With the output, it is possible to create a distribution of travel times, and thus evaluate the quality of multi-modal connections not only by the shortest possible travel time. The frequency of departure times can be tuned by setting the access_frequency parameter.
quiet	Boolean. If FALSE, display progress information on screen. Defaults to FALSE.
access_frequency	At what frequency should access trips be allowed to start? Value should be given in seconds. Smaller values will provide more accurate results, since access trips can in reality be started at anytime, but also increase processing time. Defaults to 60, i.e. every minute.
grid_access_speed	Speed at which the straight-line distance between the origin location and its nearest grid point can be travelled. Only considered when include_grid_access = TRUE. Value should be given in m/s. Defaults to 5 / 3.6, i.e. 5 km/h.
router_timeout	Cumulative travel time in seconds after which the routing algorithm will exit the search for a route. Hence, if during the search for a route between the selected origin grid point and a transit stop (see step 2 in Details) the cumulative travel time exceeds this value, the algorithm will stop looking for a connection and marks the particular transit stop as unreachable. Defaults to 60 * 60, i.e. one hour.
day	Day of the week for which the timetable should be created. May be given as unambiguous string (e.g. "tu" and "th" for Tuesday and Thursday), or an integer between 1 = Sunday and 7 = Saturday. If NULL, the current day will be used, except when date is not set to NULL.
date	Date for which the timetable should be created. Should be given as a single 8-digit integer representing "yyyymmdd". This parameter is meant to be used with feeds that contain a calendar_dates table, either to specify service availability separately for each date, or to list exceptions to the regular service availability as specified in the calendar table. Ignored when day is not set to NULL.

Details

Travel times are calculated as with the following steps:

1. The nearest grid point to origin is found. An imaginary pre-leg of the trip accounts to some extent for the distance between these two points. This distance (as-the-crow-flies) is assumed to be travelled at a fixed speed, specified in the grid_access_speed parameter. By default its value is set to a walking-like speed of 5 / 3.6 m/s, i.e. 5 km/h. When setting include_grid_access = FALSE the distance between the actual origin location and its nearest grid point is not accounted for and the trip is assumed to effectively start at the nearest grid point. Keep again in mind that no matter what approach you choose, results get less accurate with a coarser grid.

2. For each transit stop listed in the stops table of x, the shortest multi-modal travel time from the selected grid point to that stop is calculated. This is done by a purely timetable-based routing algorithm. Access trips listed in the access table of x that originate from the selected grid point are included in the transit timetable by modelling them as direct transit trips from the selected grid point towards reachable transit stops, operating on a frequency-based schedule. The frequency of these trips is defined by the access_frequency parameter, which by default is set to 60 seconds, i.e. one departure every minute. The length of such an access trip is the minimum travel time for a given access connection among all different modes that are present in the access table. The modes can be subsetted by setting the access_modes parameter, e.g. access_modes = "walk" to include only access times by foot. Possible trips itself may be filtered as well, by setting a maximum allowed travel time through the access_limit parameter. During the transit leg of the trip it is possible to transfer from one vehicle to another using a transfer connection listed in the transfers table of x. The travel time of such a transfer connection is the minimum travel time among all different modes that are present in the transfer table. The modes can be subsetted by setting the transfer_modes parameter, e.g. transfer_modes = "walk" to include only transfer times by foot. Possible trips itself may be filtered as well, by setting a maximum allowed travel time through the transfer_limit parameter. For efficiency reasons, the routing algorithm in this phase has an upper bound defined, meaning that if the cumulative travel time of a trip from the selected grid point exceeds this upper bound before the transit stop is reached, the search for a route to that stop is terminated and the transit stop is marked as unreachable. This upper bound can be defined through the router_timeout parameter and defaults to 60 * 60 seconds, i.e. 1 hour.

3. For each destination grid point (i.e. all grid points except the one selected as origin) listed in the grid table of x, multi-modal travel times from the selected origin grid point to that destination grid point are calculated. This is done by extracting travel times of egress trips that lead to the destination grid point from the egress table of x, and adding those to the travel times from the origin grid point towards transit stops as calculated in the previous step. For a destination grid point this will often result in a set of multiple travel times (since there may be multiple egress trips leading to the same grid point), of which only the shortest one is preserved. The travel time of an egress connection is the minimum travel time among all different modes that are present in the egress table. The modes can be subsetted by setting the egress_modes parameter, e.g. egress_modes = "walk" to include only egress times by foot. Possible trips itself may be filtered as well, by setting a maximum allowed travel time through the egress_limit parameter.

4. For each destination grid point listed in the grid table of x, the shortest multi-modal travel time (as calculated in the step above) is compared to the travel time of a single-modal direct trip listed in the direct table of x. The travel time of such a direct connection is the minimum travel time among all different modes that are present in the direct table. The modes can be subsetted by setting the direct_modes parameter, e.g. direct_modes = "bike" to include only direct times by bicycle. Possible trips itself may be filtered as well, by setting a maximum allowed travel time through the direct_limit parameter. The minimum between the multi-modal travel times and the single-modal travel time is the returned shortest travel time from the selected origin grid point to the destination grid point.

Value

The grid table of the GTFS-Multi feed with an additonal column containing for each grid point the calculated travel time from the given origin location to that grid point. Grid points that could not be reached get a travel time value of NA. If iterate = TRUE, there will be multiple travel time columns instead of only one. Each column then represents the travel time for a fixed departure time.

See Also

gtfsrouter::gtfs_traveltimes()

triply-at/gtfsmulti documentation built on Dec. 24, 2021, 1:21 a.m.