Virtual Mobility – simulation for optimum solutions

No matter if the goal is increasing the efficiency of existing transport facilities or planning new ones – computer simulations have become an indispensable tool in the professional life of mobility experts. But to make sure that the virtual representation of the road users’ interaction actually provides the right answers, they need one thing in particular besides high-performance software: a lot of experience.

Not so long ago the pocket calculator was among the traffic planner's most important tools. And the days of the good old deterministic planning methods are not quite over yet because some questions can still be satisfactorily solved by simplifying the scenario to such a degree that calculations based on mathematical formulas produce the required results. However, the higher the complexity of the road network, especially in fast growing urban agglomerations, the more the architects of mobility tend to profit from the new options and resources of our digital age. Using modern computer simulations, even traffic scenarios involving a multitude of interacting factors can be reproduced so realistically that there are hardly any surprises left to be faced on the way from theory to practice.

Depending on the starting situation and the goal of the investigation, two main modeling methods are used: macroscopic and microscopic simulation. The first method treats traffic as a homogeneous mass, somewhat like water flowing through a pipe. The other looks in detail at the motion of each individual vehicle-driver unit. At this microscopic level, the software calculates every little thing that happens on the highway, tenth of a second by tenth of a second. Who is overtaking whom? Who changes lane and when? The program takes account of the type of vehicle as well as of the temperament of individual motorists. The computation of the interactions between different vehicles is based not only on technical constraints, but also and above all on the as-modeled capabilities of humans as drivers. This individualized approach naturally results in far more precise forecasts of future traffic situations than an undifferentiated calculation.

Apart from efficiency, the outstanding strength of computer simulation is its versatility, as a quick look at the most important applications will show. Traffic planning is one of the disciplines that profit strongly from the possibility to put their concepts to the virtual test in a wide range of tasks, for example:

• Feasibility studies: The early evaluation of greenfield construction projects increases investment and planning security.
• Redesign of traffic-light controlled intersections: The investigation and visualization of signal plan performance and traffic flow provides political authorities and others with an optimum decision-making basis.
• Forecast scenarios: A stress test of existing traffic control systems, for instance when faced with changes in traffic density, enables the early identification of weaknesses and makes it possible to initiate and evaluate appropriate measures.
• Guidance systems and traffic routing: Preliminary investigations provide a precise assessment of the impact of the measure on the overall system.

Also when optimizing existing traffic infrastructure and road networks, the digital glimpse into the future can save the experts the one or other trip down the wrong track, for instance in the following fields:

• Strategy management: Simulation allows test runs of different strategies without any negative impact on real-life traffic and road users.
• Adaptive network control systems and traffic lights control plans for intersections or road networks: Virtual tests of the systems' control performance and environmental impact instantly reveal any potential for optimization.
• Environment-sensitive traffic control systems: Detailed mapping of traffic-related pollution and the effects of environment-sensitive control measures provide an optimum basis for defining suitable traffic guidance and control measures required for compliance with the relevant directives and master plans.
• Traffic deficit analysis: The combination of different simulation applications helps detect potential weaknesses in the transport network.
• Changes in transport services: The holistic representation of the traffic impact of events such as road works and accidents, including the current traffic situation on the surrounding road network, enables the authorities to initiate effective situation-specific counter-measures.

But this is by no means the end of the possible applications of traffic simulation. Just think of the possible uses in research and teaching institutions, for example for testing new approaches or compiling study materials, where simulation is used in particular as a source of virtual data in quantities that would be impossible to collect purely from field studies. Simulations can also make a valuable contribution to optimizing routing recommendations for traffic information services. The Bavarian Traffic Information Agency (VIB), for instance, uses the reliable predictions of traffic developments to make travel time forecasts for journeys that will start at a later time.

At the time when real road users and not their avatars used to be the guinea pigs, the authorities had to be much more cautious during the trial-and-error phase of their traffic planning and optimization projects. Accordingly, one of the key advantages of simulation is without doubt the possibility to test even quite ambitious scenarios without any risk. For the authorities responsible for mobility today, carrying out such tests in a real-life setting is virtually excluded because the dynamic challenge - especially in urban agglomerations - of accommodating fast rising traffic demand within the existing infrastructure usually leaves very little room for experimentation.

The first step of a simulation project is always the calibration and validation of the traffic flow model used. In other words, the model must be proven capable of correctly representing the actual situation. The most important criterion for simulating motorway traffic, for instance, is that there must be agreement between the real and the virtual fundamental diagrams, i.e. the development of speeds in dependence of traffic density. This is the fingerprint of any given section of motorway, so to speak. As long as the simulation is based on actual measured data, either from permanent counters in the case of motorways or from inductive loops in urban areas, there is a solid base. But the more one has to rely on assumptions already at this stage, the thinner the ice becomes. No matter how bewitching the animation, one should never allow oneself to be fooled when this is the case.

"In the wrong hands, a simulation can be an elegant way of misleading an audience," says Professor Dr. Werner Brilon, former chairman of the Scientific Commission of the German Federal Ministry for Transport, Construction and Urban Development. "Even a single minor error in one of the many input parameters may, under certain circumstances, lead to completely false results. So the extent to which I believe what appears on my monitor is a matter of trust - of trust in the skill, expertise and experience of the person who is using the model in question."

In the right hands, however, simulation may at times even be more reliable than reality - at least in a certain sense, as Professor Brilon explains: "When optimizing traffic signal controls - which is at the moment the most important application of micro-simulation - carefully planned and executed models present a very small risk of error. In fact, I would even go so far as to say that in the case of discrepancies between the simulated results and those actually measured later, this generally has less to do with the quality of the simulation than with the fact that the physical engineering measures carried out do not correspond entirely to the original plan."