Network Analysis

Last updated on 2026-10-25 | Edit this page

Estimated time: 85 minutes

Overview

Questions

  • How can a road map be represented as a graph?
  • How do we download and visualize road networks from OpenStreetMap?
  • How do we find the shortest route and measure travel time?
  • How do routes turn into measures of access, and what can go wrong?

Objectives

  • Explain nodes and edges in a road network
  • Download a drivable network with OSMnx and inspect it as GeoDataFrames
  • Compute shortest paths by distance and by travel time
  • Measure travel time from many locations to one point, the basis of an access study
  • Choose a network type and an access measure that fit the research question, and recognize the assumptions behind them

Why network analysis?

Straight line distance is misleading: a store 1 km away across a river may be a 6 km drive. Network analysis measures distance along roads, which is how people actually travel.

Road networks are stored as graphs:

  • Nodes are intersections and dead ends
  • Edges are road segments, each with a length (and, once added, a speed and travel time)

Common uses include food and health care access, emergency response times, and transportation planning.

Diagram of a small road network with eight intersections (nodes), the start A, the end B and six numbered ones, joined by road segments (edges), each labeled with its length in meters. A one-way street is drawn with an arrow. A dashed gray bracket under the network marks the straight-line distance from A to B, 700 meters. A thick orange line follows the edges from A to B along the shortest route, labeled network route, 1,150 meters, because a river with only one bridge forces a detour.
A small road network drawn as a graph.

Thinking in graphs

A graph is a simple idea: a set of points (nodes) and the connections between them (edges). In a road network, nodes are intersections and dead ends, and each edge is the stretch of road between two of them. Everything else is stored as attributes of the edges:

Edge attribute Example Used for
length 212.4 (meters) Shortest path by distance
highway residential, primary Road type, and a default speed
maxspeed 30 mph Travel time, where speed limits are mapped
oneway True Which directions can be traveled
travel_time 18.3 (seconds) Fastest path, once added

Three details matter for analysis:

  • Direction. Streets can be one-way, so OSMnx stores roads as a directed graph: a two-way street is two edges, one in each direction. The route from A to B can therefore differ from the route from B to A.
  • Weights. The “shortest” path depends on what each edge costs. With length as the weight you get the shortest route; with travel_time you get the fastest. You can also build your own weights, for example a walking cost that penalizes steep or busy roads.
  • Simplification. OpenStreetMap draws a curving road with many small points. OSMnx removes the points that are not intersections, so each node is a real decision point and each edge keeps its full curved shape and length.

Choosing the network type

The network_type argument decides which roads and paths are kept:

network_type Includes Direction
"drive" Public roads open to cars One-way rules apply
"walk" Streets and paths pedestrians can use, including footpaths where they are mapped; no motorways Every edge can be walked both ways
"bike" Streets and paths where cycling is allowed One-way rules apply

Pick the network that matches the people you study. A household without a car experiences the walking or transit network, not the driving one, and the two can give very different answers about access.

Callout

The same analysis in QGIS

The Case Studies module’s grocery access case study measures shortest paths and service areas to supermarkets in Lafayette and West Lafayette with QGIS’s network tools, without writing code. Doing both is a good way to see what OSMnx does behind the scenes.

If you have not used OpenStreetMap tags before, the Data Collection module’s Geocoding episode introduces them.

Draw a small graph on the board (four intersections, five roads) before showing any code. Learners grasp nearest_nodes and shortest_path much faster once they can picture nodes and edges. Note that nearest_nodes takes longitude first.

1. Download a road network

PYTHON 

import osmnx as ox
import networkx as nx

G = ox.graph_from_place("West Lafayette, Indiana, USA", network_type="drive")
print(G.number_of_nodes(), "nodes,", G.number_of_edges(), "edges")
ox.plot_graph(G, node_size=5, edge_color="gray", bgcolor="white")

network_type can also be "walk" or "bike". The graph is held in memory as G.

2. Look at nodes and edges as tables

PYTHON 

nodes, edges = ox.graph_to_gdfs(G)
edges[["name", "highway", "length"]].head()    # length is in meters

3. Add speeds and travel times

PYTHON 

G = ox.add_edge_speeds(G, fallback=40)   # km/h used where a road type has no speed data
G = ox.add_edge_travel_times(G)          # adds travel_time in seconds to every edge

4. Find the shortest route between two points

PYTHON 

start = ox.nearest_nodes(G, X=-86.9112, Y=40.4247)   # longitude, latitude
end = ox.nearest_nodes(G, X=-86.9290, Y=40.4580)

route = ox.shortest_path(G, start, end, weight="travel_time")
ox.plot_graph_route(G, route, route_color="red", node_size=0)

5. Measure the route

PYTHON 

route_edges = ox.routing.route_to_gdf(G, route)
print(round(route_edges["length"].sum() / 1000, 2), "km")
print(round(route_edges["travel_time"].sum() / 60, 1), "minutes")
Challenge

Challenge 1: Shortest or fastest?

Compute the route between the same two points with weight="length" and with weight="travel_time". Are they the same route? Which is longer in kilometers?

PYTHON 

for w in ["length", "travel_time"]:
    e = ox.routing.route_to_gdf(G, ox.shortest_path(G, start, end, weight=w))
    print(w, round(e["length"].sum() / 1000, 2), "km",
          round(e["travel_time"].sum() / 60, 1), "min")

The fastest route often uses faster main roads and can be a little longer in distance.

Challenge

Challenge 2: Travel time to everywhere

Use nx.single_source_dijkstra_path_length(G, start, weight="travel_time") to get the travel time from start to every node. What is the longest drive time (in minutes) within the city?

PYTHON 

times = nx.single_source_dijkstra_path_length(G, start, weight="travel_time")
print(round(max(times.values()) / 60, 1), "minutes")

From routes to access

A single route answers “how far is it from here to there?”. Access studies ask a broader question: how easily can people reach the things they need? Network analysis supports several common measures:

Measure Question Built from
Nearest facility How far, or how long, is it to the closest store, clinic or school? Paths from each home or neighborhood to every facility, keeping the minimum
Cumulative opportunities How many jobs or stores can be reached within 15 minutes? Travel times from each origin, counting the destinations under a threshold
Service area (isochrone) Which streets or areas lie within 10 minutes of a facility? Travel times outward from the facility (Challenge 2 is the start of one)
Origin-destination matrix What is the travel time between every origin and every destination? Paths for every pair; the basis of the measures above and of gravity models

Notebook 1c builds a nearest facility measure for grocery stores.

Getting on and off the network

Homes, stores and clinics are not nodes. Before routing, each location is snapped to the nearest node with ox.nearest_nodes. This works well in a dense street grid, but check the snapping distance when places are far from a road, such as a store in a large parking lot or a rural farm. The route starts at the snapped node, not at the front door, and a point can snap to a road on the wrong side of a river or highway.

PYTHON 

node, dist = ox.nearest_nodes(G, X=-86.9112, Y=40.4247, return_dist=True)
print(node, round(dist), "meters from the network")   # flag large distances

return_dist reports the distance in the units of the graph. A graph from graph_from_place is in latitude and longitude, where OSMnx measures this distance in meters along the Earth’s surface.

Assumptions and pitfalls

A network result is only as good as the network and the assumptions behind it:

  • Speeds are estimates. add_edge_speeds uses mapped speed limits, the average speed of other roads of the same type, or the fallback value. It ignores traffic, traffic lights and turns, so real drive times are usually longer, especially at rush hour.
  • The map may be incomplete. OpenStreetMap is very good in most U.S. cities, but it can miss footpaths, new roads or turn restrictions. Look at the network before trusting it.
  • Edge effects. A network downloaded for one city stops at the city limits. Trips that leave the city and come back, and stores just outside it, are missed. Download a larger area than your study area (for example with ox.graph_from_point and a generous dist, or ox.graph_from_polygon with a buffered boundary), then analyze only the origins inside the study area.
  • Mode and time of day. A driving network says nothing about buses, schedules or sidewalks. Public transit needs timetable data (GTFS) and dedicated tools.
  • Distance is not the whole story. Cost, safety, opening hours and quality of service also shape real access.
Challenge

Challenge 3: Which network, which measure?

A health department asks you to find neighborhoods where older adults without cars live more than 20 minutes from a pharmacy. Which network_type would you use, which access measure, and what would you set or check before trusting the result?

  • The walking network (network_type="walk"), because the people in question do not drive. Ideally you would add public transit as well.
  • A nearest facility measure: the walking time from each neighborhood to the closest pharmacy, flagged where it is above 20 minutes.
  • OSMnx speeds are for cars, so set a walking speed yourself, for example G.edges[u, v, k]["travel_time"] = length / speed, with a speed in meters per second. Older adults often walk more slowly than the 4 to 5 km/h usually assumed.
  • Check that the pharmacy list is complete, download the network beyond the neighborhoods to avoid edge effects, and check the snapping distances of homes and pharmacies.

Math

A network is a graph \(G = (V, E)\), where \(V\) is the set of nodes (intersections) and \(E\) the set of edges (road segments). Each edge \(e\) has a weight \(w_e\), such as length or travel time. The shortest path between two nodes is the route whose total weight \(\sum_{e \in \text{route}} w_e\) is smallest. OSMnx finds it with Dijkstra’s algorithm.

Dijkstra’s algorithm works outward from the start like a ripple. It keeps the best known cost to every node, always settles the unvisited node with the lowest cost next, and then updates the costs of that node’s neighbors. When the destination is settled, its cost is final. This only works when every weight is zero or positive, which is true for lengths and travel times.

Key Points
  • Road networks are graphs of nodes (intersections) and edges (road segments)
  • ox.graph_from_place downloads OpenStreetMap roads for any named place
  • add_edge_speeds and add_edge_travel_times let you route by time instead of distance
  • nearest_nodes (longitude first), shortest_path and route_to_gdf cover most routing tasks
  • Network distance is more realistic than straight line distance for access studies
  • Match the network type to the people you study, and choose an access measure (nearest facility, cumulative opportunities, service area) that fits the question
  • Check snapping distances and edge effects, and remember that speeds are estimates without traffic

Notebooks

Level Notebook What you will do
Beginner 1a. Road Networks: Nodes and Edges Download, plot, inspect and save a road network
Intermediate 1b. Shortest Paths and Travel Time Route between two points and map travel time across the city
Advanced 1c. Travel Time to Grocery Stores Route from every OSM grocery store to the town center