## 5 TraSLowPeR: Experimental Performance Comparison against Other Traffic Surveillance Protocols
We considered penetration rates of 100, 50, 20, and 10%. Figure 2 shows the traffic indicator \"number of vehicles\" determined by the different protocols at the variation of the penetration rate. The results obtained by the different solutions are very close, with little distance from the real number of vehicles in any case. This uniformity in the output of these different protocols depends on the relatively low vehicle density inside the target areas, which settles itself at about 80\(\div\)90 nodes. For example, the performance of the capture-recapture and the Jackknife estimator protocols is similar when the vehicle density is not such to fill the wireless medium with the status messages, which is also verified in [17]. As expected, the number of nodes is directly proportional with the penetration rate: this correlation can be clearly seen in the figure, where the indicator values determined for the 50% rate are approximately half of the ideal number of present vehicles (the same applies to the other cases). Figure 2: Total number of nodes determined over time by the protocols for different penetration rates in the two tested scenarios. Figure 3 shows the experienced message collisions suffered by the nodes for the different protocols and in the complete (100%) penetration scenario, which is the worst case for congestion, of course. The capture-recapture and the Jackknife estimator protocols, which rely on periodic status messages sent by the nodes at random instants with a frequency of 10 Hz, show a substantially higher collision rate if compared with the other solutions. While this can represent a significant problem in high node density areas, these messages are usually used by other ITS services, such as for active safety applications; therefore, when these traffic surveillance solutions are used in conjunction with other applications, they can use the existing communications to collect traffic data without introducing a high amount of new transmission overhead. The other two solutions have a comparable behavior in terms of message collisions, which is constantly lower than the other protocols. This also indicates the effectiveness of our original channel control policies to minimize the overhead of TraSLowPeR. In any case, if the RSU detects an interference with any critical applications, such as the active safety ones mentioned above, by acting as a local centralized controller it can lower the frequency used in inspecting the intersection, so to target to achieve the desired probability of collision. As confirmed in Figure 2, regardless of the protocol used, the number of nodes has demonstrated to be unsuitable to represent a meaningful estimation of the traffic status in a realistic scenario where the penetration of the connected vehicles is typically unknown. From these evidences, we concluded that it was desirable to focus our attention on different traffic indicators, to find which estimations are less adversely affected by the variation of scenario characteristics, in particular of the penetration rate in the considered scenario. In fact, the average speed of the vehicles has demonstrated to offer a more significant, valuable, and stable representation of the monitored traffic status, as depicted in Figure 4, since it has a low dependence on the used penetration rate. In particular, the figure reports the average speed of nodes traveling on two roads leading to the intersection: it is easy to notice the periodic succession of the red and green phases of the traffic light, which are characterized by decreasing and increasing values, respectively. While the average speed computed over every vehicle in the target area is not a very significant estimation, this value cumulatively computed for each single  
Figure 3: Total number of collisions over time in the most challenging case of 100% penetration rate. road converging to the intersection can instead be used as a valuable and useful indication of the volume of the traffic flow.