Routing across modes

duckosm multimodal joins the walking, cycling and driving networks of a database into one graph, so that a trip can change mode on the way: walk to the car, drive, walk from the car. How to run it and route with route_multimodal: Route across modes. Experimental.

What it adds

It writes a schema mm into the database (or the build does, with multimodal.enabled: true). It needs walking and at least one other mode; without them it only warns and writes an empty mm.transfers. mm.edges is a view, but mm.transfers is a table: run duckosm multimodal again after rebuilding a mode.

Table	Contents
mm.edges	a view of every mode's edges, with a mode column: mode, edge_id, source, target, cost_s, length_m, highway, name, geometry. The key is (mode, edge_id), because the same road has the same edge_id in every mode
mm.transfers	one row per change of mode: node_id, from_mode, to_mode, cost_s, kind

A transfer is made at every node that walking shares with another mode, in both directions. A node_id is the OSM node id (or a virtual node, node_id < 0), the same in every mode, so a shared node is the same junction.

From → to	kind
walking → driving / driving → walking	park / retrieve
walking → cycling / cycling → walking	bike_park / bike_unpark

There is no transfer between driving and cycling: every change of mode goes through walking.

Monaco: walking shares 1,382 nodes with driving and 3,786 with cycling, so mm.transfers has 10,336 rows (duckosm multimodal monaco.duckdb).

The model

A point in the graph is a pair (node_id, mode). An edge of a mode moves you inside that mode; a transfer moves you to another mode at the same node. Everything is in seconds, so a trip's time is the sum of its edges' cost_s plus the sum of its transfers' cost_s.

By default (enforce_sequence=True) a trip has this shape:

walk*  (drive* | cycle*)  walk*

It starts and ends in start_mode and end_mode (both walking by default), has at most one vehicle leg, and gets on and off that vehicle on foot. With enforce_sequence=False any sequence of modes the transfers allow is possible.

Transfer cost

Each transfer costs the same number of seconds: 60 by default, --transfer-cost on the command line. In a config file the costs can differ by direction:

multimodal:
  enabled: true
  transfer_s: 60                   # every change of mode
  transfer_costs:                  # optional, per direction
    "walking->driving": 60
    "driving->walking": 30

duckosm multimodal takes only the one --transfer-cost; costs per direction need a build with the config above.

Limits

• A vehicle is waiting at every shared junction. Transfers aren't limited to car parks or bike stands, so a trip can "pick up a car" anywhere. Good for comparing modes, not a real trip planner.
• No turn restrictions. route_multimodal routes over nodes, not over the graph of legal turns, so a driving leg can take a banned turn. For one mode, use route(), which obeys them.
• No public transport. OSM has stops and lines but no timetables.
• The graph is held in memory: fine for a city.