The Road Closure Impact Analyser fuses three data layers to simulate how a planned closure propagates through the network: TomTom probe speed data for link-level travel time conditions, SCATS signal controller data for intersection saturation and control delay, and the Bureau of Public Roads (BPR) volume-delay function to estimate how redistributed traffic changes travel times on detour routes.
TomTom Probe Data — Links
Anonymised GPS probe traces provide median speed, P85 speed, travel time index (TTI), and Buffer Time Index (BTI — reliability). Before-state link conditions are sourced from TomTom Traffic Statistics API with hourly resolution.
TTI = free-flow spd / current spd
BTI = (P85 TT − median TT) / median TT
SCATS Signal Data — Intersections
SCATS cycle-by-cycle detector counts provide approach volume, degree of saturation (DoS), and green ratios. After-closure intersection saturation is recomputed proportionally to the volume increase, capped at DoS 1.20 per SCATS overflow model.
DoSafter = min(DoSbefore × Vratio, 1.20)
d = C(1−g)² / [2(1−g·DoS)] + d₂
BPR Volume-Delay Function
Diverted volume is redistributed to detour links via proportional assignment weighted by route attractiveness (inverse of length × TTI). The BPR function then translates increased volume to travel time, quantifying added delay in vehicle-minutes per hour.
tafter = tfree × (1 + 0.15 × (v/c)⁴)
Added delay = Δt × Vafter × 60 veh-min/hr
Traffic Demand Absorption: A 15% demand reduction is applied per closed link (NZTA Traffic Note 14 — trips cancelled, rescheduled, or mode-shifted for CBD closures). Remaining 85% is assigned to detour routes proportionally. · Demonstration dataset: typical Auckland CBD weekday conditions.