  Shared spaces aim to reduce the dominance of motor vehicles by promoting
pedestrian and cyclist activity and minimizing segregation between road users.
Despite the intended scope to improve the safety of vulnerable road users, only
few works in the literature focused on before after safety evaluations, mainly
analyzing changes in users trajectories and speeds, traffic volumes, and
conflict counts, which, while useful, cannot univocally quantify road safety.
Here, we propose a more advanced methodology, based on surrogate measures of
safety and Extreme Value Theory, to assess road safety before and after the
implementation of a shared space. The aim is to produce a crash risk estimation
in different scenarios, obtaining a quantitative and comprehensive indicator,
useful to practitioners for evaluating the safety of urban design solutions. A
real world case study illustrates the proposed procedure. Video data were
collected on two separate days, before and after a shared space implementation,
and were semiautomatically processed to extract road users trajectories.
Analysis of traffic volumes, trajectories, speeds and yield ratios allowed to
understand the spatial behavior of road users in the two scenarios. Traffic
conflicts, identified with an innovative surrogate measure of safety called
time to avoided collision point, TTAC, were then used to estimate a Lomax
distribution, and therefore to model the probabilistic relationship between
conflicts and crashes, eventually retrieving a crash risk estimate. Results
show that the analyzed shared space was able to significantly reduce the risk
of crashes, and these findings are consistent with the observed changes in
users speed and spatial behavior. The analyzed case study and its limitations
were useful in highlighting the methodology main features and suggesting
practical prescriptions for practitioners.
