# Designing a C-ITS Communication Infrastructure for Traffic Signal Priority of Public Transport
## 4 Discussion  
The overall potential of the C-ITS services is derived from an intelligent combination of the individual services TSP and GLOSA. Below is a brief description of how the C-ITS services work, assuming that U5 receives 100% prioritization for public transport vehicles. This case is only given if the boundary conditions, which are defined by other traffic flows, are low. Figure 5 shows this optimal case for the TSP service. No matter where the vehicles are located and when--i.e., completely independent of their location--prioritization takes place. The drivers are thus not informed about the optimal speed or optimal stop time. As soon as the passenger exchange is complete, the traction unit approaches an intersection at its target speed and receives a clearance signal. As previously described, this reflects the optimal scenario and optimal business case. This is not possible everywhere, especially in cases of dense traffic and other traffic conditions or even political framework conditions. Examples include the prioritization of  
Figure 5: Representation of C-ITS service TSP with full prioritization: (**a**) direction south–north with TSP at both intersections; (**b**) direction north–south with TSP at both intersections; (**c**) both directions. pedestrians and cyclists or the prioritization of other conflicting public transport vehicles. In this case, it makes sense to overlay the C-ITS services TSP and GLOSA to enable optimal traffic flow within the overall public transport acceleration concept (see Figure 6). Figure 6a shows a typical trip out of town. Since the U5 trains move in the secondary direction, 100% public transport prioritization is not possible. The driver of the public transport vehicle and, in this case, the driver of a train, are now prompted with the visualization of the following information once he/she exits the tunnel:  
* Stop: The information on remaining times can be used for an optimized layover at the stop, which is used for passenger changeovers. * Route: On-route information about the optimum speed for approaching the traffic lights can be used to pass the traffic light energy efficiently when it turns green. * Traffic lights: The information on remaining traffic light phases until the next signal change can be used for an energy-efficient approach. It is not only rational but also sensible to take advantage of the residual time from previous stops and make use of this excess waiting time for an optimized and correspondingly longer passenger changeover. Doing so enables the vehicle/train to be fully prioritized at the next traffic light, where the driver can leave the stop as soon as the passenger changeover has occurred. According to the options presented above, there are different possibilities for combining the C-ITS services TSP and GLOSA. All feasible combinations of services are briefly shown and explained below in Figure 7.  
The consideration of cooperative services in the area of the C-ITS and the optimization of both the vehicles and the control of the infrastructure results in completely new objectives  
Figure 6: Representation of the combination of C-ITS services TSP and GLOSA: (**a**) direction south–north with GLOSA at first intersection and TSP at second intersection; (**b**) direction north–south with TSP at first intersection and GLOSA at second intersection; (**c**) both directions. Figure 7: C-ITS services—explanation of symbols. and questions. There is an extensive need for more research in this particular area. Some of the open questions that may have arisen will be addressed in the following paragraphs. Q1: The overlapping of the services leads to impairments in the conventional detection of traffic lights, which must still be considered separately so that the potentials can be fully exploited. The question here is: how can this interaction be optimally designed in the future? Q2: It should be noted that the quality of the forecast for the remaining time of the traffic lights and GLOSA speed recommendations has not yet been taken into account.