Glossary of Terms For Transportation, Traffic and Signalization – Chapter 1

The 15th Percentile Speed: It is the speed value which is equivalent to 15% of the vehicle speeds observed on a road segment and at the specified time after being ascending sort. In other words, 85% of drivers are driving at a higher speed.

The 85th Percentile Speed: It is the speed value which is equivalent to 85% of the vehicle speeds observed on a road segment and at the specified time after being ascending sort. In other words, 85% of drivers are driving at a higher speed.

Heavy Vehicle: Vehicles with more than 4 wheels.

Flow Rate: Equivalent value of an hourly number of vehicles, bicycles or pedestrians that crossing lanes, road or other traffic points to other points. It is called flow ratio instead of flow number because counts are not always made in 1-hour period. It is calculated by dividing the number of passing vehicles, bicycles or pedestrians into time period they pass (Usually, less than 1-hour, 15 minutes time period). Time is indicated as vehicle, bicycle or pedestrian.

Flow Proportion: It is the relative value calculated by dividing real flow proportion into saturated flow proportion for a lane group at an intersection point. 

Main Road: The road which is not controlled by “stop” signs that have priority of way at a two-way “stop” controlled the intersection

Analysis Period: A single time frame in which capacity analysis is performed at a highway facility. If the demand exceeds capacity during an analysis period, consecutive analysis periods can be selected to consider the starting sequence of previous analysis period. It is also called as “time period”.

Distance Period: Assume that there are two consecutive vehicles on the road. The time in seconds it takes for the front bumper of the second vehicle to reach the distance of the front bumper of the first vehicle.

Arterial Road: Primarily serving the traffic going straight and signalized road that has 3 km or less signal range.

Start Loss Time: The additional time consumed by the first few vehicles in seconds at the beginning of green light time period at a signalized intersection.

Rest Stage: Time spent by vehicles or pedestrians, from the moment of stopping to the moment of moving.

Bicycle: It is a two-tire vehicle ride by one person. 

Bicycle Lane: Designated part of the road by road paints for preferential or private usage of the bike.

Bicycle Road: It can be found at the left or right of vehicle roads. Facilities that not shared with pedestrians.

Build-Up Factor: Based on the growth rate of current traffic, the factor used to predict future traffic demand.

Coincidence Movement: Currents that crossing each other at an intersection.

Overlapping Approach: The opposite approach of issued approach at a stop-controlled intersection from all directions.

Bottleneck: The road element in which demand exceeds capacity by physical reasons such as accidents, vehicle incident, road works, lane narrowing, toll booth, etc. or nonphysical reasons such as road forks, stopping/standing/parking, etc.

Loop: Full series of signaling phases.

Loop Period: Total time period of a signal when it is completed the loop. 

Saturation Flow: It describes the number of vehicles within the heavy traffic flow for a specific lane group at signalized intersections. In other words, it can be described as the number of vehicles that can pass when the signal head is green for one hour at the intersection.

Roundabout: It is a signalized intersection circulating on a central island and where vehicles entering from each direction.