Looking at the bridges on either side, there is no support for the suggestion that traffic simply rerouted due to the Hartington restrictions. Counts increased by less on the nearest bridges than the next nearest, and for all bridges the percentage traffic increase is significantly less than seen in either Richmond or Hounslow as a whole:
You claim we have "little idea" of pollution levels in the absence of monitors even if we know the number of vehicles travelling on our streets. But you are stating quite the opposite when you said "of course pollution is down" on the High Road because of lower traffic, and the monitor readings confirm that this time you are correct.
We do indeed have a pretty good idea of pollution levels on the basis of traffic counts, and there can be little doubt that fewer vehicles travelling across Chiswick Bridge has helped improve our air quality. Furthermore there is no sign that has come at the expense of simply shifting the pollution across neighbouring bridges up or downstream.