Evidence Over Ideology: A Quantitative Assessment of LTNs and Blanket 20 mph Regimes in London

Abstract

We evaluate the road safety implications of Low Traffic Neighbourhoods (LTNs) and blanket 20 mph speed limits in London using injury-collision records (2020-2024). Because London implemented 20 mph limits primarily via sign-only orders without physical traffic calming, this work evaluates the blanket statutory model rather than lower urban speeds in principle. For LTNs, we apply a causal pipeline combining naive pre/post comparisons, Empirical Bayes shrinkage for Regression to the Mean (RTM), and spatial Difference-in-Differences. Naive comparisons suggest substantial crash reductions, but Empirical Bayes adjustment reveals much of this reflects mean reversion. After correction, only one zone exhibits a statistically significant inside-area reduction. We find no consistent evidence of boundary-road crash displacement nor uniform inside safety gains. Given the modest corrected benefits, small journey-time increases on boundary roads (approximately 10 seconds per vehicle) monetarily negate safety gains. For 20 mph versus 30 mph roads, five associational methods across 14 contexts show that the aggregate difference in the fatal or serious (KSI) collision share is negligible (0.6 percentage points). Controlling the false discovery rate, four contexts display robustly higher KSI shares on 20 mph roads: A-roads, junctions, pedestrian crossings, and single carriageways, with no context showing a robust reduction. In movement corridors where road geometry cues higher speeds, sign-only limits fail to self-enforce. These findings highlight a structural "wrong-road problem." Indiscriminate sign-only limits impose economic drag without proportionate safety returns. Transport authorities should move away from blanket statutory defaults and return to targeted physical engineering on high-harm corridors.