Abstract
It has been recognized since early 1980's that some of tunnels as well as vertical alignment sags in Japanese motorways tend to be capacity bottlenecks. This paper reports several facts observed at these bottlenecks and presents our hypothesis on the reason and the mechanism of the low capacities. Capacity values which are considerably lower than the usual capacity values of normal sections at the start of congestion, become even lower once the queue forms and further lower as the queue grows longer. This paper suggests that the driver's car following behavior whick determines the capacity seems to relate to the time duration that the drivers have spent in the queue as well as to the light conditions.
