In 2002 an article was published confirming what many transport professionals suspected, that when roads are closed or road capacity is reduced, the traffic is not all displaced onto surrounding roads; some of it "disappears". The study drew on 70 case studies from across 11 countries; it found a wide variation around an average traffic reduction of 11%. In trying to explain why this occurs the authors drew on 'expert opinions', which suggested that changes in mode of travel, destinations, frequency of trips and car sharing all contribute. But which of these changes occurs where, when and to what extent? That study did not attempt to answer those questions (the conclusions called for more research) and 15 years later we still don't really know.
These types of question are too simple to interest funders of academic research and too broad for individual project evaluations. But without this evidence, transport authorities are working in the dark when considering traffic removal projects. If suitable funding can be found we would like to address these questions. I would be interested to hear from any authorities who are working on projects that might provide useful case studies of area-wide traffic-removal.