Give every transport policy a public before-and-after air-quality scorecard

The problem: cities launch bus lanes, low-emission zones and street closures with strong claims about air quality, but few publish measurements that let anyone check.

The proposal: for any transport policy above a set size, require a published scorecard. Measure air quality for several months before the change, use the same sensors for a year afterwards, and compare with a similar street that did not change, so that weather and seasons do not get the credit or the blame.

Who acts: the transport department commits to the scheme; the environment department owns the measurements; an independent reviewer, such as a university, checks the method.

Cost and trade-offs: sensors and analysis are cheap compared with the scheme itself. The risks are noisy data and the temptation to measure only what looks good, so the method must be written down before the results arrive.

First step: pick one planned street scheme this year and publish the measurement plan before construction starts.