## 3 Traffic Data Sensing and Gathering
### Machine-to-Machine Communications  
Machine-to-Machine (M2M) communications technology is often named to be a promising enabler for reliable and fast data sensing and gathering [33; 34]. It has attracted focused attention from both  
Figure 3: Communications chain of data feeds in smart transportation. academia and industry to advance the data collection applications in different environments [35]. Following the recent forecasts by the leading telecommunications companies [36; 37; 38; 39], an unprecedented growth is projected for the M2M devices over the following decade--billions of nodes will be able to utilize the communications channels offered by the M2M technology. Currently, around 5 billion wireless devices are connected to various WSNs and there are forecasts telling that this number will grow to 50 billion of connected objects by the end of this decade [40]. In M2M communications, sensors (measuring devices) collect the required data and send it over the wireless links (3G, 4G (Long Term Evolution, LTE)) to a remote central server for further processing [41]. The ability of the M2M devices to avoid multihop transmission makes data delivery faster and more reliable, thus becoming a key advantage comparing to the legacy WSNs. Furthermore, the M2M technology will significantly enhance the accuracy of data sensing and gathering as well as lead to a more flexible deployment of sensors and embedded devices. We continue by reviewing some of the available technologies for M2M communications. M2M connectivity over LTE networks is expected to become a key consideration in the future TMSs. The M2M devices equipped with LTE transceivers are capable of communicating with a remote server (that aggregates and collects the received data) in a reliable, fast, and thus extremely efficient manner. Furthermore, the native support of Quality of Service (QoS) classes enables effective collection of prioritized data from multiple sources as well as guarantees the needed QoS levels for each of the data streams. However, deploying massive M2M nodes as an alternative to the currently installed WSNs will incur additional costs related to the use of cellular (3G, 4G, and 5G (emerging)) networks [42; 43]. Therefore, the attractive ICT choices and associated frequency bands are being reviewed, see Table 1 where a description of Low-Power Wide-Area (LPWA) technologies is given. Fueled by the large numbers of M2M devices (in-vehicle sensors), Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure (V2I) communications are expected to play an important role in the development of TMSs across future smart cities [45]. To ensure the required levels of efficiency for this new type of connectivity, the reliability of a WAVE system together with the IEEE 802.11p MAC protocol [46; 47] has to be taken into account in relation to the information dissemination (i.e., routing protocols). Generally, the routing protocols utilize either the (i) road network map; (ii) vehicle mobility model; (iii) both of them; or (iv) neither of them to determine an end-to-end route between the source and the destination vehicles (and sensors therein). A summary on the routing protocols for V2V/V2I communications is offered in Table 2. As shown in Figure 3, the routing protocols transfer data from the vehicles via the sensors to the gateway (often named the Machine-Type Communication Gateway (MTCG) [48]). At the gateway side, the data is sent (usually employing cellular technology, such as 3G or 4G (LTE)) to a remote traffic management controller (TMC) [49]. Then, the relevant TMC information can be used for traffic modeling as described in the following Section 4.