HEADWAY MODELING IN NORTHERN IRAQI TWO-LANE HIGHWAYS

Headway is the time interval between passages of successive vehicles past a point on the highway. More than 8000 car headways were observed in 10 level and rolling terrain two-lane highways in northern Iraq to find the critical headway and headway distribution and modeling for these highways. It is found that the critical headways are ranges between 2.5 to 4.0 seconds with a suitable critical headway of 3.2 second. The best headway model consists of composite model, for free vehicles it is a shifted negative exponential distribution, while for constrained vehicles it is a lognormal distribution.