Independent analysis of 4 Scottish LEZs using difference-in-differences with diurnal fingerprinting. March 2026.
What did we find?
We evaluated all four Scottish Low Emission Zones using difference-in-differences — comparing NO2 changes at monitoring sites inside each LEZ against sites outside. We used block bootstrap for honest confidence intervals and ran the analysis separately for each hour of the day to reveal when effects occur.
Edinburgh LEZ
−6.6 µg/m³
Evening rush (16–18), p = 0.007 Daily DiD: p = 0.051 (borderline) Traffic pattern confirmed
Glasgow LEZ
−1.6 µg/m³
Not significant (p = 0.64) No effect detected
Aberdeen LEZ
−2.8 µg/m³
Overnight only (p = 0.029) Non-traffic pattern
Dundee LEZ
−1.7 µg/m³
Not significant (p = 0.38) No effect detected
The headline
Edinburgh is the only Scottish LEZ showing a clear traffic-related air quality improvement — but you need the diurnal fingerprint to see it. The daily aggregate is borderline (p = 0.051) and would be dismissed as non-significant by a conventional analysis. The fingerprint reveals the effect is concentrated during afternoon and evening hours (12:00–22:00), peaking at 18:00 with a 7.4 µg/m³ reduction (p = 0.003). Overnight hours show no significant change — confirming the effect is from reduced traffic, not weather or regional trends.
Why the others didn't show effects
Glasgow: First Bus deployed 150 electric buses before LEZ enforcement, removing ~70% of Hope Street NOx. The DiD correctly shows the marginal LEZ enforcement effect is near zero — because the buses were already clean.
Aberdeen: The diurnal fingerprint reveals an overnight signature (22:00–03:00), not a traffic pattern. This is consistent with Union Street construction works (started Q2 2024), which involve overnight road closures. Without the fingerprint, this would have been reported as a borderline effect (p = 0.087).
Dundee: The smallest LEZ zone (~0.5 km²) with the least traffic impact. Genuine null — the zone is likely too small to produce a measurable change.
One analysis, four cities, same legislation — each demonstrating a different capability: detecting a genuine effect that a daily aggregate would miss (Edinburgh), confirming the mechanism via temporal signature (the fingerprint), diagnosing confounders that would otherwise produce false positives (Aberdeen), and explaining policy-relevant nulls with contextual evidence (Glasgow).
Monitoring Sites
Treatment sites (red) are inside the LEZ boundary. Control sites (blue) are outside. Click any marker for site details. The LEZ boundary is shown as a shaded polygon.
Site counts
City
Treatment sites
Control sites
LEZ area
Enforcement
Edinburgh
2 (EDNS, ED3)
3 (ED11, ED5, ED9)
~3.1 km²
Jun 2024
Glasgow
2 (GHSR, GLA4)
4 (GLA5, GLA6, GLKP, GLA7)
~2.6 km²
Jun 2023
Aberdeen
2 (ABD0, ABD3)
3 (ABD9, AD1, ABD8)
~0.9 km²
Jun 2024
Dundee
3 (DUN6, DUNM, DUN5)
3 (DUN4, DUN1, DUN7)
~0.5 km²
May 2024
Site classifications verified against SAQN metadata and checked against Transport Scotland LEZ boundary polygons in a spatial sensitivity analysis. Aberdeen's treatment sites are near the narrow zone boundary; conclusions are robust under strict spatial reclassification.
Data source: Scottish Air Quality Network hourly observations.
Cross-Intervention Results
Daily DiD (block bootstrap, ±365 days)
City
ρ₁
Block
DiD Effect
95% CI
p-value
PT p
Edinburgh
0.62
16d
−3.77 µg/m³ (−21%)
[−8.06, +0.02]
0.051
0.73
Glasgow
0.66
18d
−1.58 µg/m³ (−5%)
[−7.76, +4.55]
0.638
0.12
Aberdeen
0.37
9d
−2.95 µg/m³ (−12%)
[−6.08, +0.40]
0.087
0.74
Dundee
0.48
12d
−1.68 µg/m³ (−6%)
[−5.55, +2.17]
0.381
0.18
Sub-daily DiD by time-of-day window
City
Full day
Morning rush
Evening rush
Overnight
Pattern
Edinburgh
−3.77*
−3.53
−6.57**
−1.90
Traffic
Glasgow
−1.57
−1.81
−2.90
−0.65
None
Aberdeen
−2.93
−3.63
−2.72
−2.76*
Non-traffic
Dundee
−1.67
−1.67
−2.63
−0.53
None
* p < 0.05 ** p < 0.01. All effects in µg/m³. Block bootstrap, 10,000 resamples. PT = parallel-trends pre-period slope comparison p-value (higher = better; all cities pass).
Edinburgh: pre vs post diurnal profile
Figure 2: Mean hourly NO2 at Edinburgh treatment sites before (grey) and after (blue) LEZ enforcement. The afternoon/evening peak flattens post-LEZ. ±1 SD shading. Rush-hour periods highlighted in amber.
Edinburgh: effect by time window
Figure 3: Forest plot of DiD effect estimates by time-of-day window. Evening rush shows the largest, most significant effect. Overnight is non-significant.
Diurnal Fingerprints
Each bar shows the DiD effect estimate for one hour of the day. Dark blue = p < 0.01, light blue = p < 0.05, grey = not significant. Rush-hour periods shaded.
Edinburgh LEZ: clear traffic signature. 13 of 24 hours significant, peaking at 18:00 (−7.4 µg/m³, p=0.003). Overnight hours not significant. This pattern is consistent with reduced through-traffic on the Nicolson Street corridor during afternoon/evening commute.
Glasgow LEZ: flat fingerprint. No significant effect at any hour. Consistent with bus fleet electrification (150 electric buses deployed 2020–2023) having already cleaned up the dominant emission source before LEZ Phase 2 enforcement.
Aberdeen LEZ: overnight signature. Significant reductions at 22:00–03:00 but not during rush hours. This is NOT a traffic pattern — consistent with Union Street Central construction works (started Q2 2024) involving overnight road closures and traffic diversions.
Dundee LEZ: no significant effect at any hour. The smallest zone (~0.5 km²) with limited traffic impact. One borderline hour (13:00, p=0.07) but no coherent pattern.
How It Works
1
Load hourly NO2 from UK regulatory monitoring networks (AURN, SAQN) via the Aeolus data library. Filter to ±365 days around each LEZ enforcement date.
2
Classify sites as treatment (inside LEZ boundary) or control (outside). No boundary buffer exclusion due to sparse Scottish networks.
3
Aggregate to daily zone means. No deweathering applied — we found that daily-resolution deweathering suppresses intervention signals regardless of training strategy (documented in detail). Raw DiD is the recommended estimator for daily data.
4
Difference-in-differences with block bootstrap (10,000 resamples). Block length auto-estimated from autocorrelation using Lahiri (2003). This produces correctly-sized CIs — validated at 90,000 Monte Carlo trials showing 93–100% coverage at ρ ≤ 0.6.
5
Diurnal fingerprint: repeat step 4 for each hour of the day (0–23), producing 24 effect estimates. Classify the pattern as "traffic" (rush-hour effects), "non-traffic" (overnight), or "none". This provides mechanistic evidence from monitoring data alone.
6
Interpret against policy context: bus fleet changes, construction, tram extensions. The fingerprint diagnosis + context explains each city's result.
Why block bootstrap?
Daily NO2 data has temporal autocorrelation (ρ ≈ 0.4–0.7): today's value predicts tomorrow's. Standard (i.i.d.) bootstrap ignores this, producing confidence intervals that are too narrow and a false positive rate of 24% (at ρ = 0.6) instead of the nominal 5%.
Figure 4: Monte Carlo validation (90,000 trials). Block bootstrap (blue) achieves correct coverage; i.i.d. bootstrap (red) fails severely at ρ ≥ 0.6. Green band = target 93–97%.
Why no deweathering?
We tested three deweathering approaches for daily DiD and found all suppress the intervention signal. The root cause: daily aggregation destroys the within-day variance that deweathering preserves. The DiD already controls for common weather trends — the same city's weather affects treatment and control sites equally.
Policy Context
Edinburgh — the tram and the LEZ
Edinburgh's Tram to Newhaven extension opened 7 June 2023 — one year before LEZ enforcement (June 2024). This is a pre-period change that improved the baseline before the LEZ took effect, making our estimate conservative. The LEZ effect we detect is on top of any tram-driven improvements. The George Street redesign and City Centre Transformation programme (£314M) may also contribute, but Nicolson Street (our primary treatment site) is not part of that redesign.
Glasgow — the bus fleet story
First Bus deployed 150 BYD ADL electric double-decker buses to their Caledonia depot, progressively from 2020 through March 2023 — before LEZ Phase 2 enforcement. Buses were responsible for ~70% of NOx emissions on Hope Street (our primary treatment site). The UBDC study (Shin et al. 2024) found Hope Street NO2 fell 9–13% from fleet change alone, with no change in traffic volume. The DiD correctly shows the marginal enforcement effect is near zero.
Policy insight: The LEZ announcement and grants programme may have been more effective than enforcement itself, by motivating fleet investment.
Aberdeen — construction on LEZ streets
Union Street Central construction started Q2 2024 — an 18-month project directly on the LEZ's main street, coinciding exactly with enforcement. New market building construction from February 2024 (£20M). South Harbour expansion opened September 2023. The overnight diurnal signature (22:00–03:00) is consistent with construction traffic management — overnight road closures diverting traffic away from treatment monitors.
Dundee — smallest zone
At ~0.5 km², Dundee's LEZ is the smallest of the four. Xplore Dundee deployed 12 new electric double-deckers (smaller scale than Glasgow's 150). The null result is most likely explained by the zone being too small to affect enough traffic to produce a measurable change at monitoring sites.
Shared confounders
ScotZEB: The Scottish Zero Emission Bus Challenge Fund funded electric bus deployment across all four cities. Glasgow received the largest allocation (150 buses), others smaller numbers. This is a shared confounder that biases all DiD estimates toward zero.
National fleet turnover: UK-wide diesel vehicle scrapping and EV adoption affects all sites equally. Captured by the DiD design (common trend assumption).