When it comes to traffic, the recipe for evaluating a piece of infrastructure such as an intersection seems to be relatively simple. You use the information in the relevant design manuals, the number of vehicles passing the intersection, you assign the volumes to the different movements and you then take a reading from the existing tables and diagrams of the average waiting times for road users. The evaluation is now complete. But what happens if the traffic signals are vehicle actuated or if the junction is part of a road network with complex interactions with other intersections? In cases like these, microsimulation makes a real difference.
“If it is done well, simulation is an extremely powerful tool for conveying a message”, says Paul Speirs, Technical Director at WSP, a worldwide leading engineering consultancy. “Advising clients, stakeholders and the public, who are generally not experts in transport planning, is a fundamental part of our work. When facts and figures become tedious, we find simulations can quickly and visually set the context showing the pros and cons of transport measures.” This is how it worked in Northampton, UK.
Northampton, a traditional English market town, has ambitious plans for sustainable economic growth over the next 20-30 years by building more homes and creating more jobs, both of which inevitably generate more traffic. So how do you deliver growth without adding to an already congested town? “All modelling must have a purpose and in this case it’s all about unlocking development”, says Speirs. Potential development sites can sit idle for years because the cost of infrastructure improvements often outweighs the benefits of the development. “We have used our Northampton town centre Vissim model, which has over 40 signal controlled junctions, to test numerous strategies, junction improvements and development impacts and the model has now become a crucial and integral part of redesigning the highway network.”
Being able to co-ordinate and optimise so many infrastructure improvements in a holistic manner has become fundamental in setting out a coherent strategy. Speirs adds, “without doubt Vissim’s dynamic real time visual simulation has underpinned the wider transport strategy and decision making has become so much easier. Through our solid modelling evidence base we have secured central government funding to enable delivery of infrastructure improvements that truly have unlocked development potential. It’s that simple really, which is the whole point. The results are literally there to see.”
Simulation is a hands-on skill
The construction of roads, roundabouts instead of intersections, metering and route guidance within the scope of traffic management measures – everything that has an impact on the traffic flow – can be simulated. “One of the main use cases is definitely the simulation of signal control systems”, says Dr.-Ing. Axel Leonhardt, Director of Product Management PTV Vissim. “And as a special application: traffic-actuated signal control. For the latter, microsimulation is particularly well suited because here, as in reality, individual vehicles trigger signal controls.”
On the basis of schematic maps of intersections and aerial photographswhich include infrastructure elements such as parking lots, stop lines and the position of traffic signal heads, the transport planner makes a model of the network. This is where care and attention pay off: with the ‘links and connectors’ concept used in Vissim, networks can be modelled in a high level of details, a prerequisite for realistic simulation of traffic flow. In order to model signal control systems, the user reproduces the control logic, either from scratch directly in Vissim or by linking an external controller with Vissim using an interface. Once the supply side has been dealt with, the demand is taken care off. Data from automatic or manual traffic counts or from strategic transport models such as PTV Visum are used as input for the simulation model. In addition to “normal “ road traffic, Vissim can also simulate public transport, cyclists, pedestrians and other modes of transport. A genuine multi-modal traffic simulation system evolves, where different transport modes are not only simulated alongside each other but in a trulyintegrated manner. Subsequently, the model has to be calibrated so that it can be adapted to local driver behavior and vehicle characteristics. “It is at this level of modelling that we can establish driving behaviourthat reflects reality. With parameters the modeler can define for example the drivers’ aggressiveness and what that means for their lane changing behaviour”, explains Leonhardt.
Modelling driving behaviour
In order to be able to realistically reproduce route choice, lane choice, lane changing and car following behaviour, various models are implemented in PTV Vissim that can be tailored to the user when it comes to local area parameters. Professor Wiedemann’s psycho-physical car following model considers four driving states: With ‘free driving’, the driver can move along at their individual desired speed. If they are approaching a slower vehicle in front of them, they change to the ‘approaching’ mode. “The driver recognizes that there is a slow moving vehicle in front of him and brakes early on such that the distance to the leading vehicle will not become too small”, says Leonhardt. The third mode is the ‘following’ mode. Here, the driver tries to maintain a steady distance from the vehicle in front. “Typically, the driver will not be able to perfectly do so”, explains Leonhardt. “Often, the distance between the two vehicles oscillates – sometimes the speed is slightly higher, sometimes slightly lower. The traffic planner can adjust the degree of oscillation using Vissim.” If the vehicle in front decelerates suddenly due to something happening, the vehicle following must also ‘brake’. “For each vehicle, Vissim checks each simulation time step, how large the distance is and the differences in speed in relation to the vehicle in front.” In this way, the software can establish whether a vehicle should decelerate or accelerate.
And what about lane change? “Here too, we have implemented models that simulate lane change behaviour”, says Leonhardt. In addition to free lane change, which takes place when overtaking, for instance, there is necessary lane change which comes about because the driver wishes to follow a particular route and therefore must branch off, for instance. “A special feature of Vissim is that we can also map lateral movement within a traffic lane. This is important, for instance, when modelling cyclists, motorcycles or rickshaws, as these do not drive around in single file, instead they also drive alongside each other”, says Leonhardt, adding that, “this type of flexibility is one of the characteristics that sets Vissim apart from the competition.”
This post is also available in: German