What Bologna’s data shows
For two and a half years, Bologna’s Città 30 initiative has been one of the most debated urban mobility measures in Italy. Debated, precisely, because the conversation unfolded almost entirely without solid evidence, driven by first impressions, assumptions and political opposition. The data is now available. And some of the answers cut through the noise.

Four million trips. One rigorous methodology
Conducted in collaboration with the City of Bologna and the Fondazione IU Rusconi Ghigi, the study draws on big data from vehicle black boxes: over 4 million trips across the Bologna urban road network, analysed to answer the questions that have dominated the public debate.

The analysis is built on two Floating Car Data (FCD) datasets: an aggregated set covering 2023 to 2025, with twelve sample weeks per year, tracking speeds, travel times and critical driving events; and a high-frequency dataset with GPS sampling at one-second intervals, covering over 135,000 trips and more than 90 million sampling points. This second, high-resolution dataset underpins the emissions modelling.

Three core questions drive the analysis: how much time do drivers lose? Have emissions increased? Are the roads safer?

Travel times: less than you think
The median travel time across the entire urban network increased from 3’32” per kilometre in 2023 to 3’58” in 2025, an overall rise of 12.6%. Read in isolation, that figure might appear to validate the critics. The deeper analysis tells a more nuanced story.

When roads affected by the 30 km/h limit are compared separately with roads that remained at 50 km/h, the largest slowdowns are found on the latter (+28 seconds per km, +14.8%). Routes typically travelled by an average urban driver show the smallest increase: +14 seconds per km, equivalent to just over one minute on an average trip of 5.2 km.

The overall increase in travel times across the urban network is largely attributable to external factors, most notably construction work for the new tram lines.

To isolate the pure effect of the 30 km/h limit, the researchers ran a theoretical simulation: assuming all vehicles in the sample were electronically constrained to respect the limit, travel times would increase by a maximum of 5% compared to times actually recorded, amounting to just 24 seconds on an average trip of under 10 minutes.

Emissions: the data confirms what urban traffic analysis already indicated
Estimates calculated using an emissions model based on COPERT Tier 3 show a reduction across both climate-altering and pollutant emissions: CO₂ -14.6%, CO -17.8%, NOx -23.7%, PM -21.7%. To those unfamiliar with real-world urban driving dynamics, this may seem counterintuitive, since a combustion engine is generally more fuel-efficient at 50 km/h than at 30 km/h in terms of consumption per kilometre. For those who work with actual traffic data, the explanation is well established.

The key lies in driving behaviour. Travelling at 30 km/h in a more consistent and uniform manner produces significantly fewer harsh stop-and-go events, which are the most energy-intensive and emission-heavy phases of any urban driving cycle. Sudden accelerations and hard braking, more frequent at higher speeds in urban environments, generate consumption spikes that translate directly into elevated emissions.

When vehicle fleet renewal between 2023 and 2024 is also factored in, the combined effect produces even more marked reductions: CO₂ at 95.3%, CO at 91.3%, NOx at 82.6% and PM at 84% relative to the October 2023 baseline. This comparison shows that, in terms of quantitative reduction of pollutant and climate-altering emissions, the impact of the Città 30 measure is comparable to that achieved through fleet renewal alone.

Road safety: the numbers speak clearly
Accident data recorded by the Bologna local police confirm a widespread reduction in collisions and injuries following the introduction of the measure, with road fatalities halved and pedestrian deaths reaching zero.

The resulting saving in social costs from road accidents approaches 66 million euros for the City of Bologna, based on a conservative estimate using parameters from the Italian Ministry of Transport updated to 2025.

The probability of a harsh driving event resulting in a collision fell by 22.7% in 2024 and by 17.2% in 2025 compared to 2023. On roads covered by the 30 km/h limit, extreme braking events more than halved.

A side effect that cannot be ignored
The analysis also surfaces a critical finding that deserves attention. On roads outside the Città 30 zone, harsh braking events increased by 30.5% in October 2024 compared to October 2023. The researchers interpret this as a form of road rage: frustration built up in slower zones is released, once drivers leave them, through more aggressive driving behaviour.

Speed moderation achieved through physical road design, including lane narrowing, chicanes and raised intersections, tends to produce more stable compliance and less negative psychological reactivity than regulatory imposition alone. This reinforces the importance of accompanying infrastructure investment.

The regulation, on its own, is not sufficient. Physical traffic calming is the component that will determine the long-term effectiveness and durability of the measure.

The European picture: Bologna is not an outlier
A systematic review covering 40 European cities quantifies average reductions of 23% in accidents, 37% in fatalities and 38% in injuries following the introduction of 30 km/h speed limits.

In Wales, the 20 mph limit triggered street protests and a petition with nearly half a million signatures. Two years later, the data shows 630 fewer collisions, and the implementation cost was recovered within the first year through savings on the social costs of road accidents. The pattern recurs across contexts. Opposition tends to be loudest at the outset. When the data arrives, it tells a different story.

What Bologna teaches us
The Bologna case, from the TAR administrative court ruling of January 2026 that annulled the original measure, to its reintroduction through 22 ordinances covering 258 km of roads in April 2026, is more than a story about urban policy. It is a story about how decisions get made without evidence, challenged without evidence, and ultimately evaluated with evidence.

The GO-Mobility analysis demonstrates that measuring the real-world effects of a mobility policy is achievable, with the right methodology and the right data sources. The question every city administration should be asking is not “are residents for or against this?”, but “what do the data show on safety, emissions and travel times?”

Designing safer and more sustainable cities requires monitoring tools that match the scale of the decisions involved. That applies to speed limits. It applies to low-emission zones. It applies to sustainable urban mobility plans. It applies, more broadly, to any intervention that aims to produce measurable and lasting change in how people move through cities.

Source: https://datamobility.it/magazine/bologna-citta-30-top-o-flop/