# Managing Emergency Situations in VANET Through Heterogeneous Technologies Cooperation
## 6 Performance Evaluation
### Proposed Architecture Evaluation  
Our proposal is compared with the common protocol used in VANET environments for signaling warning to others vehicles and another protocol based on cloud services where ITS runs its tasks to reassign traffic load around the covered area. We have to point out some considerations that have driven our simulations. First of all, collisions between vehicles may happen in the closest areas around the cross-road junctions. We considered the E-OBU able to rise warning when an obstacle is found to the vehicle. To have comparable results between our proposal and other ones, the simulated vehicles are equipped in other environment with the same OBU. As consequence, the number of protocol messages increased, but it permits to reduce the number of pileups. In the first campaign, we propose system behavior versus the increasing of vehicle density in the monitored area, taking as fixed parameters the ones shown in Table 4.  
First of all, it is important to evaluate how the ITS system installed in the higher architecture layer impacts on traffic conditions. Regarding ITS implementation and its behavior we implemented a solution already proposed in the [57] where the _Ad-hoc On-demand Distance Vector_ (AODV) protocol is used to disseminate ITS messages in the VANET environment. Therefore, when the ITS term is reported, we refer to the proposed model in [57]. In Figure 15(a), the trend of average time that a vehicle may spend in the city is depicted. In order to perform this simulation campaign we consider two kinds of collision. The first one is called medium severity. Here the average duration of a collision is 10 min. In the second case, called high severity, the average collision duration is 20 min. It is shown that, increasing the updating time related to the map weights, the traveling time increases. This means that ITS manager reacts more slowly with the increasing of update time. In this scenario, we evaluate the capability of the system to respond at collision events. In this figure it has been demonstrated that the higher is the collision severity in terms of duration the higher is the traveling time experienced by vehicle. This happen because system redistributes traffic density too slowly and it involves more areas. In this way, it is clear that ITS solutions only based on centralized services may not be enough to face congestion issues. In Figure 15(b), the number of collisions are reported: it is possible to note that the increasing of update interval, in the ITS core, increases vehicle density in certain area. This issue augments the probability that a collision could happen. In case of high severity the number of measured collisions are lesser than medium severity because it has been measured that the average speed is lesser than average speed measured in the case of medium severity. This happens because of vehicle queues that are longer and numerous. Figure 15: Travelling Time versus Flow Density. In this last scenario the ITS centralized approach is compared with the distributed and multi-layer approaches presented in this paper. In Figure 17a the trend of traveling time (that represents the time spent by a vehicle in the city area covered by ITS service) is represented. It has been demonstrated that, in the case of the distributed approach, it is possible to reduce the queue waiting time by approaching faster to traffic issues, such as collisions or congestions. This allows system to respond faster to these events by rerouting vehicles and avoiding local congestions. In Figure 17b the average traveled distance per vehicle is depicted for centralized and decentralized ITS solutions. Here it is shown that the decentralized approach allows vehicles to travel a shorter distance than the centralized ITS, because of the possibility to find an alternate path when something is recognized by local infrastructure. In the case of centralized solution, instead, vehicle rerouting can be performed only after the complete updating of the map, with the current road status.