“Many traffic jams could be avoided”
Interview with Justin Geistefeldt
by Julia Weiler
March 26, 2015
Mr Geistefeldt, you have developed a model for estimating the reliability of German motorways for the Federal Transport Infrastructure Plan 2015. What does that mean?
Reliable traffic flow means that the travel time on a certain route is more or less the same every day. Unexpected incidents, on the other hand, mean that road users have to allow for more time if they want to reach their destination likely on time. Reliability should be considered when evaluating the construction or widening of roads (see “Calculating the reliability of motorways”).
The A 40 in the Ruhr region – one of Germany’s most congested motorways – is currently being widened from four to six lanes between Bochum and Essen. Will traffic jams thus become manageable?
After widening it to six lanes, there will be fewer traffic jams. However, a portion of the traffic jams will relocate to other bottlenecks, e.g. the junction Dreieck Essen-Ost. Moreover, the traffic demand may change following the road widening. At present, many road users are prepared for traffic jams, use other means of transport, choose alternative routes, or set off before or after the rush hour. Once the road widening is completed, it’s possible that more people will be using the A 40 during peak hours. Thus, it is not to be expected that congestions will completely disappear.
What are the goals when planning such a road widening?
Traffic quality is rated using a school-grade system. As the evaluation concept originated from the USA, the grades are A to F, with A being the top grade. When planning a road, the goal is not to reach level A. It would be uneconomic to make all roads wide enough to achieve excellent traffic quality. One usually aims at quality level D. It’s the same as in a written exam: many students simply want to pass. The road authority can also specify the target value to be C or only E. Generally, however, it will be D.
That does not sound particularly ambitious.
You have to consider how the level of service is defined. It applies for one assessment hour, the so-called 30th hour. A year has 8760 hours. The traffic volumes in these hours are sorted in descending order. The assessment of traffic quality is based on the traffic volume reached in the hour ranking #30. But that does not mean that traffic quality in the annual average represents level of service D.
In addition to road widening, what else can be done to make a motorway more reliable?
Fast clearance of accidents, effective work zone management and traffic control. Traffic demand can only slightly be influenced, but we can try to homogenise traffic to prevent a traffic breakdown.
Traffic breakdown means congestion.
That’s right. Whether or not a traffic jam will occur depends, among other things, on the drivers who use the road at any given moment. On one and the same route, there may be a traffic jam occurring at a traffic volume of 3900 vehicles per hour one day, and the next day there won’t be one despite 4100 vehicles per hour. The heavier the traffic, the more vehicles drive in platoons. If the vehicle ahead brakes, the driver of the car behind will brake as well – but only after a short reaction time. In order to compensate for the reaction time, he has to brake a bit harder, and the third one has to brake even harder and so forth. At some point, one of the vehicles will have to stop. This is how a traffic jam builds up.
Are there any driver types that particularly cause less congestion?
Commuters who know their route usually drive more efficiently than, for example, holiday-makers. Consequently, many motorways in metropolitan areas have a higher capacity than motorways in holiday regions – even if they have the same road geometry. Commuters know that it is pointless to change into the left lane when driving short distances. They may only be driving to the next exit or the one after that, and they simply stay in the right lane. They thus systematically create a smoother traffic flow than on long-distance or holiday routes, where the speed differences between the lanes are greater and traffic distributes more unevenly between the lanes.
In such cases, traffic has to be homogenised.
Precisely, for example by using line control systems with variable signs which warn of danger ahead or display traffic dependent speed limits. They are used to reduce the speed differences between the lanes so that nobody will have to brake too hard, thus causing a traffic jam. Ramp metering helps, too. Traffic lights in entry ramps that release only one vehicle per green time ensure that vehicles do not enter the motorway in platoons. In heavy traffic, one or two vehicles may be able to merge smoothly, without causing a traffic breakdown. If several cars come in all at once, they may cause a traffic jam.
In NRW, these measures have been long implemented. And yet, it remains Germany’s federal state with the highest traffic jam percentage. What else can be done?
The work zone management has still potential for optimisation. When planning a work zone, a compromise between the optimal solution for the construction process and the optimal solution for the traffic flow must be found. Currently, decisions are made at the expense of traffic flow quality in too many cases.
The interview was conducted by Julia Weiler.