TRAFFIC FLOW CHARACTERISTICS ON MAIN ROAD 4 BETWEEN LAHTI AND HEINOLA IN FINLAND

The average daily traffic on the 25 km long three-lane road section between Lahti and Heinola on main road 4 is over 12,000 veh/day and the summer average daily traffic is over 15,000 veh/day. During weekend peaks the traffic is very congested for several hours. Besides high traffic flows and poor road geometry, car drivers cutting in from the passing lane into the platoon in the basic lane at the end of the 2.7 km long passing lane at Vierumaki have been suggested to be the causes for the congestion. In this research work travel speeds and overtaking rates on the three-lane road and causes for the congestion during Friday peak traffic were investigated. In daytime traffic the travel speeds on the three-lane road are higher than 95 km/h in both directions of (Q less than 600 veh/h). In weekend peak traffic the travel speed in the main direction is remarkably lower: on Fridays 40-50 km/h at 17:00-19:15 (1100 less than Q less than 1650 veh/h) and on Sundays 65-75 km/h at 16:00-19.30 (1,000 less than Q less than 1,750 veh/h). The decrease of the speed level is observed on different road sections on Fridays and Sundays. On Fridays the road section between Ahtiala and Vierumaki is congested, on Sundays the section between Heinola and Vierumaki. The decrease of the speed level on Fridays is also affecting the overtaking rates. On Fridays the overtaking rate between Ahtiala and Vierumaki is multifold compared to the overtaking rate between Vierumaki and Heinola. The results showed that the critical section towards Heinola from the capacity point of view is the about 2 km long uphill road section with only one lane in the uphill direction just before Vierumaki interchange. The average grade of the uphill is 1.5%. The steepest section, which is about 200 m long and has a grade of 2.5%, begins immediately after the automatic traffic measurement point (LAM 601). The speeds of the heavy vehicles decrease remarkably on the uphill road section: the lowest speeds of the trucks were 69-84 km/h on the single lane, where overtaking of slower vehicles is not allowed. From the video tapes from Friday peak traffic it can clearly be seen, that a continuous queueing begins at 16:17, after a passenger car with a caravan trailer moving uphill has passed the automatic measurement point with a speed of 75 km/h. The average delay caused by the congestion on the road section between Seesta interchange and the bottleneck on the uphill road section at Vierumaki was estimated to be about 8.5 minutes. The time costs caused by the congestion on this road section were estimated to about 8,500 Euros. As a whole, the time costs of the congestion can therefore be estimated to the magnitude of about 10,000 Euros.