## 3 Our Original Traffic Surveillance Solution for Low Penetration Rate Scenarios  
As a follow-up of the relevant research results achieved within the framework of the EU COLOMBO project, we have worked to design, implement, and validate original traffic surveillance protocols with the aim of collecting a large set of monitoring data about traffic status near to intersections. The final goal is the identification (and comparative quantitative evaluation) of decentralized algorithms and protocols for cooperative traffic surveillance that are able to generate reasonably accurate values of a series of indicators, useful for effective traffic light control algorithms. According to these general guidelines and by considering, since the beginning, realistic and practical usage scenarios for actual traffic optimization, we have designed an original local protocol called Traffic Surveillance for Low Penetration Rate (TraSDowPeR): the TraSDowPeR collection areas are limited to the transmission range of each RSU node, located at relevant intersections; in fact, in our preliminary work, we established that the limited traffic monitoring data in the RSU coverage area are sufficient to obtain meaningful indicators about traffic status near intersections, while the additional information obtained in the expanded coverage area provided by multi-hop routing protocols were not necessary to the aim of traffic surveillance for our local traffic light optimization system [20]. In our targeted use cases only few communication-enabled vehicles are typically inside their respective transmission range, so we deemed appropriate the definition of a direct communication protocol between RSUs and all the vehicles inside their transmission range, with the goal of creating a very detailed view of the traffic flows at the central point of the RSU for each intersection locality. Given the focus on locality of our TraSDowPeR solution, we discarded the use of collaboration between vehicles for the collection of the traffic information (examined in previous papers such as [20; 21], since we believed redundant the use of clustering techniques to group the nodes together for the traffic surveillance and traffic light management application domains. We also observed that, in our typical use cases, the average cluster dimension would be close to one, since the size is directly proportional to the penetration rate (low in our challenging assumptions), making the exploitation of clustering techniques unessential. Furthermore, we believe counter-productive the use of data aggregation mechanisms among mobile nodes because of the limited quantity of data available that TraSLowPeR requires to transmit (see below); instead, we preferred the transmission of every information, locally available on board of each vehicle, to the RSU, which can be further refined with data fusion practices to create high-level indicators inside the RSU, if needed. However, since our direct transmission mode can require large bandwidth if used in high-penetration rate scenarios, we have carefully reduced any communication management overhead by adopting a simple coordination technique to avoid transmission surges that can briefly saturate the channel, leading to high collision rates. This is achieved through two different mechanisms. On the one hand, we limit the transmission opportunities of vehicles by restricting them to send messages only as a reply to an appropriate request received from a RSU (otherwise, they are always kept in an idle status). On the other hand, we separate in time and space the communications of mobile nodes traveling on the different roads leading to the considered intersection, thus reducing the sources that can simultaneously transmit. This separation is obtained by specifying an exact targeted road and direction in each request message sent by the RSU, which will be taken into consideration only by the appropriate nodes.