Here is a copy and paste of the full conclusion section, with emphasis added, for those who don't want to read the whole thing.
Conclusions
Mean falls in motor traffic on internal roads are around ten times greater than mean rises in motor traffic on boundary roads, adjusting for background trends. We believe that this result suggests that these LTN schemes may be contributing to ‘traffic evaporation’ or ‘disappearing traffic’ (
Cairns et al., 2002). Traffic evaporation refers to the many ways in which people adjust habitual behaviour in response to restrictions, which may lead to a reduction in motor traffic. This may simply mean a car trip to a specific destination is directly replaced by a walking or cycling trip. Probably more common, however, are more complex changes: for instance, a person makes fewer trips to the supermarket (by car) and more shopping trips on foot to the local shops, or combining car trip destinations to increase efficiency.
8 We cannot say which of these types of behavioural response is dominant in the schemes studied here.
Specifically, this study found substantial reductions in motor traffic within scheme areas, while across boundary roads there was
very little aggregate change (+0.7% mean average compared to background trends). We have not attempted to calculate overall traffic reduction due to these schemes, because aggregation is affected by the number of count points, and in most cases, more counters could have hypothetically been placed (particularly on internal roads, more numerous than boundary roads). However, the results indicate that motor traffic has been reduced, and only a small proportion re-routed to boundary roads. This is suggested by the mean increase of 82 vehicles per day on each boundary road being much lower than the mean reduction of 815 vehicles on each internal road.
This indicates that LTNs can contribute to wider traffic reduction objectives. It is encouraging to see on average so little change in boundary road traffic volumes. Perhaps the widely fluctuating Covid-19 trends (as well as the tendency of some authorities to introduce schemes in September following the annual August fall in congestion) have contributed to perceptions that such schemes make large contributions to boundary road traffic. And it may still be the case that despite relatively little changes in boundary road traffic, those car journeys which continue to be made as before take longer simply because cut-throughs are no longer available. This study only looked at motor traffic volumes, not journey times or trip distances (e.g. through a travel survey); different methods are needed to test and quantify changes in journey time or trip distances. For instance,
Goodman et al. (2023) used Driver and Vehicle Licencing Agency data, which found a 6.4% decrease in distance driven per vehicle among those living in areas in Lambeth, South London, where LTNs were introduced.
Finally, it is important that boundary roads are not forgotten. They do experience often substantial traffic burdens, and just over half the boundary roads in this study saw increases over the monitoring periods (with just under half seeing a reduction). Tools are needed to reduce burdens on boundary roads, whether by traffic reduction and/or mitigation of its effects. A paper by
Hajmohammadi and Heydecker (2002) found that the introduction of London’s Ultra Low Emission Zone in April 2019 led to reductions in NO, NO2, and NO
x concentrations both within the implementation zone and the wider low emission zone (LEZ) and Greater London area. LTNs represent one tool alongside many others (such as bus priority, pollution taxes, road charging, and/or car parking levies, which address different aspects of the problem and/or address different behavioural patterns) that may be needed to achieve the ambitious reductions in car use sought by many cities including London. Further research could explore and compare the impacts and different contributions of these tools alone or in combination.
