An interpretation of a traffic engineer on vehicular traffic flow

The present paper focuses on capacity of tunnels and sags on motorways. Free flow becomes congested due to speed reduction caused by sudden change of light condition at tunnel entrances or by unrecognized increase of gradient at sags combined with drivers' behavior that they do not shorten their car-following spacing corresponding to the speed reduction, resulting in reduction of flow rate of successive vehicles. The free flow to congested flow breakdown takes place at the flow rate level of around 3,000 vph per two-lane (one direction). Once congestion queue is formed, the capacity value is determined by drivers' behavior of departing from the queue at its front-end. This departure flow rate is in the range of 2,200 to 2,700 vph per 2-lane and varies depending on time during which the driver has been caught in the queue, light condition (day or night) and some other factors.