Higher-order synergies identify connected cycling backbones in Hamburg street network

The study combines the city's empirical street graph with a perturbed utility route-choice model calibrated on observed bicycle flows. It treats joint upgrades as higher-order interactions: upgrading one link raises the marginal benefit of upgrading a topologically complementary link along the same path while lowering the benefit of redundant parallel links. Computationally, this is expressed as the mixed partial derivative of total network performance with respect to pairs of edge capacities. The resulting synergy matrix is thresholded to extract a sparse backbone of mutually reinforcing corridors.

Standard betweenness or single-link marginal-gain rankings miss these pairwise effects and therefore favor scattered upgrades that produce little cumulative travel-time reduction. In Hamburg the synergistic backbone concentrates on a small subset of radial and tangential segments whose coordinated improvement yields up to 2.4 times the system-wide benefit of an equal budget spent on the highest-ranked individual links. The approach directly addresses the policy reality that cities rarely upgrade single links in isolation.

Because the framework relies on a static demand model and assumes upgrade costs are additive, it underestimates dynamic mode-shift responses and construction sequencing constraints. Validation against longitudinal ridership data after actual corridor projects would strengthen external validity. Extension to multi-modal networks and inclusion of equity-weighted demand remain open implementation steps for planners.