6. Transit Accessibility of all locations in the Region
This section explains the Transit accessibility → Region part of the Accessibility Calculator menu (Figure 1).
Figure 1. Transit accessibility → Region menu
We present in detail the From every location – Fixed-time departure option and then the differences for each of the three other options.
6.1. The necessary datasets
Transit routing database, see Construct Databases → Transit routing database.
The layer of buildings opened in the current QGIS project.
6.2. Accessibility from every location in the region, fixed-time departure
Choose Transit accessibility → Region maps → From every location – fixed-time departure. Enter the parameters (Figure 2).
Figure 2: Region → From every location – fixed-time departure dialog
Enter the necessary parameters:
Transit routing database folder — the folder of the transit routing database must contain files stops.pkl, stoptimes.pkl, transfers_dict.pkl, idx_by_route_stop.pkl, routes_by_stop.pkl
Output folder — the name of the folder for storing the results of the computation.
Output alias — the alias name for the files of results and layers of visualization.
Layer of all origins in the region — the layer of the region buildings, may be selection set (if checked).
id — the field of the unique identifier of a building, in the layer of buildings.
Layer of destinations — the layer of the destinations, may be selected buildings (if checked).
id — the name of the field with the unique building identifier, in the layer of buildings.
Visualization layer — the layer that will be used for visualization of accessibility maps, must be a part of the current QGIS project.
id — the field of the unique identifier of the visualization layer feature. The identifiers must be the subset of buildings’ identifiers. More information here.
Minimum number of transfers — the minimum number of transfers of the transit trip, typically 0.
Maximum number of transfers — the maximum number of transfers of the trip, typically 1 or 2.
Maximum walk distance to the initial PT stop, m — the maximum acceptable walking distance between the trip origin and the first bus stop. The default value is 400 m.
Maximum walk distance at transfer, m — the maximum acceptable walking distance between two stops at the transfer. The default value is 150 m.
Maximum walk distance from the last PT stop, m — the maximum acceptable walking distance between the last stop of a trip to the destination. The default value is 400 m.
Start at (hh:mm:ss) — trip start time.
Walking speed (km/h) — walking speed.
Maximum waiting time at the initial stop, min — the maximum waiting time at the initial stop of the trip.
Maximum waiting time at the transfer stop, min — the maximum waiting time at the transfer stop.
Boarding time gap — the minimum time between two sequential activities, like arriving at the stop and boarding the bus. Usually, zero or several seconds.
Maximum travel time, min — the maximum total travel time.
Number of bins — the number of bins of the time interval [0, Maximum travel time]. The aggregate accessibility measures will be stored for every bin, just as the cumulative histogram frequencies.
The bin width is equal to the Maximum travel time /Number of bins, and the Number of bins must not exceed the Maximum travel time. If the last bin does not match the Maximum travel time, the measures for the Maximum travel time are also stored. Typically, the Number of bins is selected in a way to have the bin’s width of 5 or 10 minutes, while a 2- or even 1-minute bin can be useful for further analysis of accessibility.
Aggregate — each of the numeric attributes of buildings can be selected for aggregation. For example, if the number of jobs is known for a building, then the total number of jobs that can be reached from a building in a given time can be calculated as a measure of the from-accessibility to jobs. You could choose several fields to aggregate (Figure 3).
Figure 3. The choice of attributes for aggregation
The sum of the aggregated attribute over buildings achievable during the time of one, two, etc., bins will be stored as a separate file for each of the aggregated fields in the Output folder. Each of the aggregated measures is presented by the thematic map. In case the computations are based on the selection of buildings, the results will include the new layer that represents these buildings. The structure of the result files is described in the next section.
Click Run to start. The Progress bar shows the progress of the computations. You can break the process of the computations by pressing Break. The Log tab contains the parameters of the run and run’s metadata and is explained in the next section).
6.2.1. The log file and REGION accessibility report
The log file (Figure 4) is stored in the folder of the results. It lists all settings of the run and the computation time.
Figure 4. The log file of the Region → From every location – fixed-time departure computations
In the case of the from-accessibility, the basic file of results presents the total number of buildings that can be reached from each of the region’s buildings after every time bin (Figure 5):
| Attribute | Meaning |
|---|---|
| Origin_ID | The ID of the building of origin |
| One bin time | Total number of buildings accessible in 1 time-bin |
| Two bins time | Total number of buildings accessible in 2 time bins |
| … N bins time | Total number of buildings accessible in N time bins |
| Maximum travel time | Total number of buildings accessible in maximum travel time (if the latter is not an integer number of bins) |
Figure 5. The structure of the output table for the Region maps → From every location – fixed-time departure option
The thematic map presents the number of buildings reachable in maximum travel time. Additional result files present the totals of other attributes chosen for aggregation and for each of these attributes the thematic map of the result for the maximum travel time is constructed. The example of the Transit accessibility → Region maps → From every location – fixed-time departure computations here.
6.3. Accessibility to every location in the region, fixed-time arrival
To compute the to-accessibility of every location in the region run the To every location – fixed-time arrival option. Most of the parameters of the region’s to-accessibility computations are the same as for the from-accessibility. This regards walking distance, walking speed, waiting time at stops, number of transfers, and the gap between sequential activities. The major difference is in establishing origins and destinations – for to-accessibility, the region’s buildings are destinations, and one must provide the layer of the origin buildings from which these destinations may be accessed (Figure 6).
Figure 6. The part of the region to-accessibility dialog that is different from the corresponding part of the from-accessibility dialog
The Log and Result files are the same as for the from-accessibility, with minor differences that reflect the from-accessibility to the to-accessibility changes. The example of the Transit accessibility → Region → From all locations – fixed-time departure computations see here section.
6.4. Region accessibility for the schedule-based departure or arrival
The modern users of public transport are aware of the time the bus arrives at the stop they plan to start from or at the final stop of the trip. These travelers start their trip and walk to the initial stop to be there just before the bus arrival or take the bus that arrives close to the destination just before the time a traveler must be there. We have modified the RAPTOR algorithm to compute accessibility for these schedule-informed travelers. The explanation of the schedule-based view is presented in the section devoted to the facilities service area here. Just as for the service area computations, instead of one “start time” parameter, schedule-dependent accessibility from every building in the region demands two parameters – Earliest start time and Maximum delay at start, min both included in the dialog. The time between the earliest start time and the actual start of the trip is not included in the total travel time. In the case of schedule-based accessibility to every location in the region, the latest arrival time is also substituted by the arrival interval that is defined by the Earliest arrival time and Maximum lateness at arrival, min, and the time between the arrival and the latest possible arrival is not included in the total travel time either. The examples of schedule-based accessibility are here. The example section contains the comparison between the time-fixed and schedule-dependent accessibility estimates.