## 3 Space-Time Transport Performance Measure (_STTP_)  
### Key Features of _STTP_  
_STTP_ aims to measure the performances of the transport system based on the spatio-temporal and individual constraints characterising the daily life of each individual. To meet this purpose, _STTP_ grounds on three key features, described below in detail.  
* **Focus on the Individual Daily Travel Cost (_IDTC_) incurred for the daily fixed activities:**_STTP_ is not focused on the sum of the discretionary opportunities potentially reachable given the schedule of fixed activities (i.e., the _FOS_). Instead, it focuses on the individual travel cost incurred to perform the daily schedule of fixed activities (from now on named Individual Daily Travel Cost; _IDTC_). _IDTC_ is calculated as the generalised cost of transport incorporating both monetary and non-monetary cost (see Section 3.2 for further details). This shift of perspective allows _STTP_ to focus on a factor that is transport-specific rather than land-use-specific and thus isolate the performance of the transport system. Moreover, it allows _STTP_ to focus on an indicator that is suitable to represent transport-related accessibility differences. Indeed, individuals tend to minimise the transport cost needed to reach their fixed daily destinations [42; 44]. However, by focusing on _IDTC_, _STTP_ also excludes the discretionary part of accessibility typically included in _STA_ and representing the potential for activities offered by surrounding amenities. This choice requires _STTP_ to be complemented with _STA_ to capture the potential component of accessibility.
* **Weighting of _IDTC_ against the Individual Daily Distance (_IDD_):**_IDTC_ is usually influenced by the distance daily travelled: the higher the distance, the higher _IDTC_. This may be misleading for evaluating individual transport-performance differences, since even in the case of identical transport performances, people who travel longer distances would be more disadvantaged than those travelling short ones. To isolate the role of the transport system performance in individual accessibility differences, this variable needs to be controlled to exclude its influence from the analysis. For this purpose, _STTP_ weights _IDTC_ against the Individual Daily Distance (_IDD_). This is the sum of the Euclidean distances between each couple of subsequent activitiesbelonging to an individual's daily schedule (see Section 3.2 for further details). This choice makes _STTP_ a measure of transport performance rather than a measure of the daily transport effort of individuals (which would be influenced also by the distance daily covered).
* **Estimation of _IDTC_ based on temporal and individual constraints:** To incorporate the temporal constraints, _STTP_ calculates _IDTC_ by considering the actual location and timing of the fixed activities daily performed by an individual. Also, the individual constraints are incorporated in the _IDTC_ computation in two ways. First, the actual modal choices of individuals for each daily travel are considered according to individual constraints such as the ability to drive or car ownership. Second, the non-monetary cost part of _IDTC_ (i.e., travel-time costs) are estimated at the individual level based on income (as described in detail in Section 3.2).  
These three features shape _STTP_ as a transport performance indicator that stems from spatio-temporal and individual constraints. On the one hand, this allows _STTP_ to isolate the role of the transport system in individual accessibility. On the other hand, it suggests how _STA_ and _STTP_ should be deployed together to get both a comprehensive picture of space-time accessibility and more narrowed information on the transport component. Figure 1 summarises the relation between _STA_ and _STTP_, while Section 3.2 describes how _STTP_ is calculated.