Algorithmics for Safe Bicycle Network Design with Bounded Detours in Rural Areas

2026-08-10

AI Summary

This study addresses the problem of upgrading rural road networks for safe bicycle travel under a limited budget: given a graph representing rural roads, the goal is to minimize upgrade costs while ensuring that between every origin–destination pair, there exists a safe path whose length does not exceed α times the original shortest path distance. The problem is formalized as the Safe Bicycle Network Design with Bounded Detour (SBNBD). The authors establish its fixed-parameter tractability for the first time, using a novel parameterization combining the number of terminal pairs and the feedback edge set size. Matching lower bounds under the Strong Exponential Time Hypothesis (SETH) and kernelization hardness results reveal the critical role of structural parameters in algorithmic efficiency. Leveraging tree decompositions, parameterized algorithms, ILP formulations, and preprocessing with cut-generation based on feedback edge sets, experiments on German OpenStreetMap data demonstrate substantial improvements in solution speed and quantify the trade-off between detour factor α and upgrade cost.

Abstract

We introduce the \emph{Safe Bicycle Network with Bounded Detours} (\emph{SBNBD}) problem, motivated by upgrading rural road networks for bicycle traffic. Given an undirected graph with safe and unsafe edges, edge lengths, upgrade costs, terminal pairs, a budget, and a detour factor $α$, the task is to upgrade unsafe edges so that each terminal pair is connected by a safe path of length at most $α$ times its shortest-path distance in the original network. We study SBNBD from a parameterized perspective. We prove strong NP-hardness on restricted graph classes, including planar graphs of treewidth two, graphs with feedback vertex set number one, and graphs of maximum degree three, and complement these lower bounds with polynomial-time algorithms for trees and graphs of maximum degree two. We show fixed-parameter tractability for the number of unsafe edges and prove matching SETH-based lower bounds, a polynomial-kernel lower bound, and W-hardness for natural parameters. Our main structural result maps any instance to an equivalent instance with $O(\mathrm{fes}+p)$ vertices and edges, where $\mathrm{fes}$ is the feedback edge number and $p$ the number of terminal pairs; this yields fixed-parameter tractability for $\mathrm{fes}+p$. Finally, we evaluate ILP-based algorithms on OpenStreetMap road networks for small German municipalities and their surroundings. The instances have small treewidth upper bounds and moderate feedback edge structure. Preprocessing based on the $\mathrm{fes}+p$ reduction and tree-decomposition-based cut generation both improve exact solving, especially on harder instances. Experiments with different detour factors show that increasing $α$ can reduce the upgraded-edge length, revealing trade-offs between upgrade cost and allowed relative detours. Overall, structural graph parameters provide a useful algorithmic lens for safe bicycle-network design.

Problem

• Safe Bicycle Network
• Bounded Detours
• Graph Upgrade
• Rural Road Networks
• Terminal Pairs

Innovation

• Safe Bicycle Network Design
• Bounded Detours
• Parameterized Complexity
• Feedback Edge Set
• Graph Reduction