## 6 Discussion  
The first observation from the results is that the VTL mechanisms (both the algorithm of [14; 20] and ours) make a major difference by themselves, inasmuch as travel times and fuel consumption are significantly lower in configurations 1 and 3 than in configurations 0 and 2, notwithstanding which policy drives the traditional traffic lights in the latter. Global savings of around 5% in travel times and around 10% in fuel consumption are, undoubtedly, great incentives to develop standard VTL protocols, to be supported by the majority of the vehicles. In the comparison between configurations 1 and 3, it turns out that our proposal for VTL management attains greater savings than that of [14; 20]. The average difference is around 2% of global travel times and 3% of fuel consumption, which can be taken as a justification for the increased complexity of our stack of protocols (Figure 1) in comparison with placing VTL logic directly on top of IEEE 802.11p (It must be noted that the VaNetLayer and VNIBR protocols are of general use, so they can support any other communication services in VANETs in addition to VTL management, as explained in [12; 13]). The advantages are most noticeable with medium traffic densities, because our proposal--thanks to the virtualization infrastructure--supports multi-hop communications and persistent state information; therefore, we can manage larger VTL areas to coordinate more incoming vehicles at each intersection. The difference is not so noticeable with light traffic, because the vehicles can go through the crossroads swiftly most of the times, and so any VTL logic can work well even if some messages are lost. With heavy traffic, in turn, vehicle movements are slow enough so that more efficient communications do not bring about better VTL performance. On the other hand, we can see that platooning alone can attain travel time reductions of up to 7% under light traffic conditions, and up to 2% with heavy traffic (the aerodynamic advantages of travelling very close to the preceding car vanish when the average speed is low). However, when platoons are combined with VTLs that seek to facilitate their transit through every intersection (which is only possible with our proposal), the savings go from nearly 65% with light traffic to almost 72% with heavy traffic: there is more to gain when congestion is not an issue after each road segment. Anyway, the benefit is substantial, and it is not achieved to the detriment of cars that do not participate in platoons, as evidenced by the fact that the global averages are better in configuration 2 than in configuration 0, and better in configuration 3 than in configuration 1 as well. Looking at the figures of configuration 3 in Tables 1-3, it turns out that the combined management of VTLs and platoons is most beneficial for platoon members in cases of high traffic density, but the global savings are greater with medium density. The reason is that, in the latter case, it is easier for the VTL logic to match an incoming platoon and one or more cars interested in joining them. In congested scenarios, in contrast, those cars may still be queueing behind a few others when the platoon crosses the intersection.