of the intersection. The coverage is bi-dimensional for the inner part of the intersection, and linear for each lane of the approaches. Each grid unit may take one of six possible states representing the dynamics over one second of the road users that passed over the corresponding part of the roadway (see Figure 3). The six possible states can be aggregated into four main states: emptiness, moving presence, stationary presence and end of presence.

The data is difficult to interpret at grid-unit level. Since the spatio-temporal definition is limited, it is not possible to identify individual road users and it is difficult to estimate distances between them. To make interpretations more robust to noise, components of neighboring grid units are considered. Precisely, the neighbours of a grid unit are the four
(or two for lane approaches) closest units, i.e. the ones that are vertically and/or horizontally adjacent. The connected components are formed independently for each grid-unit state in each functional zone of the intersection. The connected components correspond to either moving or stationary groups of road users and will be referred to in this way from now on. Thresholds are further applied to the number of units in groups below which the groups are discarded (more illustrations are provided in (Saunier et al., 2003)): they were determined empirically to be 2 and 3 units respectively for moving and stationary groups of road users.

## 4 A Rule-Based Method For The Detection Of Critical Situations 4.1 Detecting Stop Line Crossings

The first step to detect critical situations and measure the exposure to lateral collision is to identify the origin of moving groups of road users detected in the conflict zone. This is achieved by detecting stop line crossings, using a method developed previously by one of the authors (Midenet, 1998). It relies on rules counting the number of grid units in some states on both sides of the stop line in order to determine the situations upstream and downstream of the stop line. This is a generic method that allows to detect crossings from one side to the other.

Two subsets of grid units, respectively upstream and downstream of the stop line, i.e.

in the storage and conflict zones, are delimited, and henceforth called only "upstream" and "downstream". The number of grid units taken into account for each zone may be adjusted in the system. For each set of grid units, the ratio τ*state* is defined as the number of grid units in the given state, denoted n*state*, divided by the total number of grid units in the set. To make the presentation of the rules simpler, two new grid unit states are built upon the four possible values:
Presence corresponds to moving presence, end of presence or stationary presence Movement corresponds to moving presence or end of presence The "movement state" of each subset of grid units is set as either Empty, Stationary, P ast Movement, F uture movement or *Movement* according to the rules, presented as algorithm 1 for the downstream zone and algorithm 2 for the upstream zone. For these states to be taken into account, the downstream and upstream zones must meet the crossing conditions described in algorithm 3, and a crossing between the two zones is detected according to the rule of algorithm 4.

Algorithm 1. Qualification rules for a downstream zone.

4