Ulrich Leth, project assistant at Vienna University of Technology, Institute of Transportation, Research Center of Transport Planning and Traffic Engineering, gave an interview for ORF's "Home in Austria" on traffic jams. Since he wasn't able to say everything he considered important in a matter of minutes, and concerns the project CHESTNUT, we bring his more extensive reflection on mobility.

What causes traffic jams? What are the consequences?

- Traffic jams are caused by overloads if too many road users are traveling at the same time (e.g. early morning traffic), or by accidents, when the capacity of the road is limited. Another source of traffic jams is inhomogeneous driving behavior (frequent acceleration and deceleration processes e.g. in the course of overtaking maneuvers) in already dense traffic. The smoother and more homogeneous the speeds, the smoother the traffic. Incidentally, the optimum speed is around 80 km/h. Any higher speed - whether it is 100, 130 or even 140 km/h - leads to a lower performance.

Why don't we just build more lanes?

- You can't eliminate traffic jams by building more roads. In the short term, there are relief effects through capacity expansion (e.g. by adding one more lane); however, they will be replenished because the attractive infrastructure will attract more traffic until a new balance (i.e. traffic jam) re-emerges. Impressive examples can be found altogether in the USA, where even 8-, 10- or 12-lane highways are stowed. But even in Vienna there is a prime example: the A23 (Tangente). Originally intended as a bypass for the city center, in the meantime, a bypass of the relief road - the S1 - was built, and yet the A23 is the traffic jam hotspot number 1 in Austria.

While in the car, you can't escape the traffic jam. The driver himself can't solve the problem because he is part of the problem (congestion).

From a scientific point of view, congestion is - quite neutrally - an indication of (too) attractive, (too) cheap infrastructure, similar to long queues in front of the cash register during winter sales.

From a traffic planning point of view, traffic jams are not a traffic problem, but one of the most effective measures (in addition to parking management) to nudge people towards a more sustainable mobility behavior. If driving by car everyday and everywhere is your fastest and cheapest transport option, you will never question your mobility behavior, never inquire whether commuting with public transport would be possible - even more convenient - and never get the idea of maybe cycling and integrating the workout into your daily commute. It's exactly this thinking process that starts in the traffic jam.

Which measures help against traffic jams?

- On the one hand, you can increase the efficiency of the existing incrastructure. In commuting, the occupancy rate is just 1.1 - that is, only in every 10th vehicle is occupied by 2 persons, the 9 others are traveling alone. This is extremely inefficient. With simple measures, however, the occupancy rate can be increased, e.g. with the promotion of carpooling. In other countries, there are already high-occupancy vehicle lanes, i.e. tracks on highways, which are reserved for vehicles with 3 or more passengers. As a result, more people with fewer vehicles can be transported in the same cross-section - and in less time, because there is less jamming.

On the other hand, alternatives to car traffic must be available. Public transport must be competitive in travel time to car traffic. No one uses public transport when it takes three times as long. But also the bicycle can relieve the road network only if the infrastructure is designed so that everyone dares to drive there. After all, 4 out of 10 car rides in Austria are shorter than 5 km - a distance that can be traveled by bike in 15 to 20 minutes, in the city often even faster than by the car, because the search for a parking spot is unnecessarily.

What will the future bring?

- Currently, there are very dynamic developments. E-mobility will become increasingly important in the future. However, this won't help a lot against traffic jams. The space requirement of an e-car is exactly the same as that of a conventionally operated vehicle. The second, even more important trend: autonomous driving. Self-driving cars will revolutionize our mobility behavior. On highways it will be soon reality (there is already a test track in Austria); within the city, it will probably take decades, because the situation there is much more complex. The chances of autonomous driving are obvious: less accidents, less congestion, mobility for everyone (even for those without a license). This could however cause an explosion of car traffic, if everyone then can drive (or even empty cars). And this is accompanied by the risk for the health care system, when even shortest routes that are currently being traveled on foot or by bike are then made by car for convenience. Furthermore, there are still many open ethical questions (especially in the case of an accident) and the issue of data protection.

Either way, it will be important to actively shape the legal and transport framework conditions so that the new technologies are used for the benefit of our society. Otherwise we could be overrun by the dynamic developments and the particular interests of the car industry.

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