Effectiveness in linear traffic management systems

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Dirk Kemper

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Real-time evaluation and improvement of the effectiveness of preventive dynamic speed limits in linear traffic management systems

Preventive variable speed limits of traffic management systems (TMS) have the aim to optimize the traffic flow concerning the traffic security and the utilization of the existing traffic capacity. According to “Merkblatt für die Ausstattung von Verkehrsrechnerzentralen- und Unterzentralen (MARZ 99)” the aim has to be achieved by the harmonisation of speed on the onehand. Another possibility of intervention is the temporary display of “no passing” concerning trucks. They have to fix the slow heavy goods vehicles on the right side lane, to keep the additional passing lanes free for passenger cars.

Today the evaluation of the upcoming variable speed limits occurs rarely, so that a check up of the parameters for the control algorithm often happens only time-delayed, i.e. offline and this also only for a small number of periods of time. A prompt valuation of TMS would allow for a permanent assessment of the goodness and quality of the upcoming variable speed limits over which a continuous statistic could be created. Resulting from this, changes and tendencies of the traffic flow could be identified.

The aim of this study consists in the determination of adequate evaluation parameters of the harmonic traffic flow and in the realisation of software components for online identification and presentation of this values in a traffic computer system. Two assessment approaches were chosen for this. The first one considers the changes of a microscopic parameter concerning the different traffic states. The effectiveness compares the actual traffic with the unaffected traffic flow. This method can be regarded as a relative concept. The second concept is based on established measurement of traffic
harmonisation. Hereby the assessment occurs not via a relative comparison with an unaffected traffic status, but rather with ideal, absolute specific values.

The last mentioned concept has been recommended for online systems, because it considers a multitude of criteria, which describe according to experts an harmonisation state and has been verified by statistical examinations.

The problem of the distinction if the regarded traffic state can be traced back to the changed variable speed limits or if it results from the traffic volume, should be considered by a two dimensional assessment concept on the basis of the assessment method developed by STEINHOFF (2003). The acceptance index of a variable speed limit describes on the one hand the speed compliance level on the most left lane and otherwise the compliance level of an optional truck passing restriction. The second value, the harmonisation index of the traffic considers the standard deviation of the speed on the most left lane, the distribution rate of the vehicles on the second and third lane (on three lane sections) as well as the dynamic quantity of the difference speed on the second lane of sequent time intervals. Both quantities together provide the effectiveness index of variable speed limits on a scale of values between zero and one. The method will be adequate to two and three lane sections.

The assessment interval has been defined to one minute, which considers the basis of a control cycle of a TMS. The TMS are based on traffic control recommendations which determined every minute from current information about the traffic volume the local traffic density and the average speed.

The assessment method will be realized by means of a software component, which will be integrated in the recent developed DaV-system by order of the German road construction authority. All significant technical functions and program units as well as the integration of the complete system has been verified in a prototype version An additional option to do an offline analysis should provide the subsequent analysis of traffic data.

The practicability of this assessment method has to be proved iteself in the practical use. Thus it is possible in the future that the chosen parameters have to be improved or extended. However the assessment concept has been determined and the software extension will be possible without great efforts. Concerning the traffic it has to be assessed that the relationship between the used values (parameters) und especially to the traffic stress only limited experience are available. Furthermore it is necessary, if further improvement of the description of the traffic within the short-term intervals should be achieved, to enforce the research efforts to that effect. A concrete description of harmonized traffic flow will be necessary in the future, to provide the evidence that a harmonized traffic flow will result in an increase of the traffic security and the traffic quality.