# Interdisciplinary Urban Tunnel Control within Smart Cities
## 3 New Road Tunnel Traffic Control Algorithm  
Urban road tunnels, when left without control, often face congestions. Congestions influence several aspects from performance index in Equation (2). Clearly, they impact the transport performance (less vehicles can get through the network), but they also negatively influence the environmental aspects.  
The congestions in tunnels can be addressed mainly by two control approaches. Both of them are based on highway management strategies [15; 16]:  
Figure 5: The cumulative cost of congestions (in CZK) and the level of service (LOS 1—green; LOS 2—yellow; LOS 3—red) as computed for an example network in Prague at (**a**) 8 a.m. and (**b**) 11 p.m.  
Figure 6: An example demonstrating the split of the cost of congestions (in CZK) to particular contributing factors Q_i during the day.  
1. Influencing characteristics of the traffic flow, usually through speed harmonization. It has been demonstrated [17] the capacity of a communication is increased in case a speed limit is imposed on highways. This is achieved through harmonizing the traffic flow (i.e., minimizing the speed differences among vehicles), thus decreasing the vehicular headways and thus increasing the throughput. Nowadays, this control approach is however limited by the existing norms (for example Czech norm 73 6101) stating that the tunnel is just a continuation of the road network and should maintain the same speeds. For this reason, a speed variation is not a suitable option.
2. Changing the travel demand parameters, particularly by managing (constraining) the number of vehicles entering the tunnel. This is similar to the concept of ramp metering, often adopted on highways and freeways.  
Nowadays, tunnels are typically controlled by the second approach. It is, however, usually performed manually and often in a binary matter. In case queues are forming in a tunnel, the police decide to close the tunnel. This principle is demonstrated on a case study of an urban road tunnel complex Blanka (Prague, Czech republic), which is depicted in Figure 7. It is an important part of the (still not finished) Prague city ring and it was built to lighten the traffic in the central historical areas of the city. The tunnel complex is 5.5 km long and it is composed by three underground tunnels (red lines in the Figure 7). The opening of the tunnel complex Blanka in 2015 had a major impact on traffic in the city and clearly demonstrated need to take such tunnel complex as a part of the city.  
The effect of tunnel closing clearly leads to an improvement within the tunnel, but has a strong negative impact on the traffic around the tunnel, in the rest of the city. This is described by [PERSON] and [PERSON] in [18] and demonstrated in Figure 8. The green points denote sensors for which the average speed increased after opening the tunnel complex Blanka; the red points mean decrease in average speed. Always, the change in percent is presented. The full circle denotes the sensor that measures in one direction only, the split circles show values in both directions.  
Figure 7: Schematic overview of the tunnel complex Blanka.  
Another negative aspect of the current traffic control is the recovery phase, after the tunnel is opened again. It can take several dozens of minutes (sometimes even hours) to recover the stable state after the tunnel is opened [19].