Transportation Engineering Civil Engineering Department 4 th Stage

Transportation Engineering Civil Engineering Department 4 th Stage, 1 st Semester, 2019 -2020 10 th Lecture: Traffic Characteristics, Density and Headways Lecturer Dr Maha Osamah Dr Abeer K. Jameel

Traffic Characteristics: Density and Headways Density �It can be defined as the number of vehicles occupying a given length of road at a given instant. �Density is usually expressed as vehicles per kilometer (veh/km). �High densities indicate the vehicles are very close to each other, while low densities imply greater distances between vehicles.

Traffic Characteristics: Density and Headways Measuring Density �It is difficult to measure the density at the field. � It can be measured through Ariel photography which is an expensive method, or �it can be estimated theoretically from the densityflow-speed relationship.

Traffic Characteristics: Density and Headways �

Traffic Characteristics: Density and Headways Headway Two types of headways describe the traffic characteristics, �Time headway and �Space headway

Traffic Characteristics: Density and Headways Time Headway (ht): �Time between the arrivals of successive vehicles at specified time. �It can be computed as the difference between the time of the front of a vehicle arrives at a point on the highway (t 1) and the time the front of the next vehicle arrives at the same point (t 2). � It is usually expressed in seconds.

Traffic Characteristics: Density and Headways �

Traffic Characteristics: Density and Headways Space headways (hs) �It is the distance between the front of a vehicle and the front of the following vehicle. �It is usually expressed in meter. �The average spacing in the traffic lane can be directly related to the density of the lane Hs=1000/D Where: D= the density of a lane, veh/km/ln hs = the average space headway (m)

Traffic Characteristics: Density and Headways Gap and lag Gap is the headway in a major stream which is evaluated by a vehicle driver in a minor stream who wishes to merge into the major stream. It is expressed in either units of time (time gap) or units of distance (space gap) 3 3 Major stream 1 Minor stream 3 2

Traffic Characteristics: Density and Headways �Time lag: is the difference between the time a vehicle that merges into the major traffic stream reaches a point on the highway in the area of merge and the time a vehicle in the major stream reaches the same point. �Space lag, is the difference, at an instant time, between the distance a merging vehicle is away from the a reference point in the area of merge and the distance a vehicle in the main steam is away from the same point. 3 3 Major stream 1 Minor stream 3 2