Harmonization with Variable Speed Limits on Motorways

Abstract

This paper presents an investigation and a state-of-the-art description of traffic effects of harmonization with variable speed limits. Expansion of the E4 south of Stockholm has been carried out during the years 2009-2013. The expansion includes a rearranged lane configuration within the existing road section (25 m) and traffic management systems. Previous design with two lanes and hard shoulder has been replaced with three lanes without shoulder. In addition, the road has been equipped with variable speed limits (with red ring), queue warning system with recommended speed and emergency refuge areas (ERAs). The queue warning functionality was activated in 2011 and harmonization in 2013. Reduced speed variance is an important goal for the traffic management system and is considered to both reduce rear-end collisions and the risk of capacity breakdown. German experience shows that harmonization affects capacity down to about 80 kph. English experience is that the harmonization and monitoring can reduce accidents by 40% at normal speed of 70 mph (112 kph). Germany and England decrease the speed with approximately 20 kph at the degree of saturation of 0.7 to let the flow be harmonized before the traffic density has increased too much. A measure to describe disturbed and undisturbed traffic can be the relative change in the standard deviation of the average speed for 5-minutes periods updated each minute, called CVS. In case of accidents, CVS may be twice as high as normal. The standard deviation of all vehicles is calculated for each minute and aggregated into five-minute periods. The harmonization settings on E4 south of Stockholm has been iteratively designed and ultimately set to 325 vehicles/5 min, which has worked well. The variable speed limit is reduced to 80 kph and stabilizes the flow. The harmonization indicates a potential risk of queue for the road users, which becomes more prepared if the queue warning is activated further along the road due low speed and the risk of sudden braking. The main results can be summarized as follows: • The average speed during rush hour on weekdays has increased by 2.5 kph after the installation of the traffic control system, of which 25% is assumed to be attributable to the traffic management system • The harmonization has delayed the onset of collapse. During the periods in which harmonization has not been activated but there has been a breakdown, it is considered that harmonisation would have passed the half period and then collapse would occurred. • The accidents have been reduced by half, of which 25% is assumed to depend on traffic management (harmonization, queue warning and VMS) The queue warning has worked well, but could be improved by also exploiting the so-called CVS. The measure reveals the instabilities in the flow, sometimes tens of minutes before the AID-alarm due low speeds is enabled. In rush hour when traffic is dense (> 325 vehicles/5 min) and harmonization has already been activated the CVS measure gives further information, which could be exploited to improve the queue warning algorithm.

Keywords

Harmonization ; Variable Speed Limits ; Motorway ; Capacity

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