Traffic characteristics. In the event that we need to think about the Traffic attributes and their expectations, it is appropriate to furnish at any rate their short posting with definitions and shared relations. The three fundamental characteristics in traffic flow theory are flow, speed, and density. Traffic flow. Traffic flow can be additionally found in some literature named as flow, flow rate or volume. Nevertheless, all these terms can be used interchangeably. It is characterized as various vehicles passing a point in a given timeframe. It is typically communicated in units of vehicles per hour vph. Other conceivable units are for example vehicles per hour, per lane, vphpl, passenger car units per hour pcu/hr, or passenger car units per hour per lane pcphpl or in easier readable form pc/h. ln The equation for counting traffic flow is simple where q is traffic flow vph n is number of vehicles passing a spot on the road in a given interval t. The special value of traffic flow is capacity c which defines the maximum hourly rate under prevailing roadway conditions. Since it is convenient to measure traffic flow in 15-minute intervals a quantity called peak hour factor PHF is presented. Hence is obvious that PHF can theoretically be in the interval 0 25 PHF 1 0. Peak hour factor can be understood as an indicator of flow fluctuations within the hour Introduction.
10 Speed Based on different approaches how to calculate speed there are two main interpretations time mean speed and space mean speed. Time mean speed is defined as the average speed of vehicles passing a spot on a road. The standard notation is ut U_t _ i 1 n u_i n where ut is time mean speed in km h ui is spot speed of vehicle i measured at a spot along a road usually by a radar or laser gun km h n is a number of vehicles. Space mean speed is defined as the average travel speed of vehicles between two points at the distance D apart. It is computed as where us is space mean speed D is the distance of two points on the road km is the average travel time h. Space mean speed is more useful in the context of traffic analysis and is determined on the basis of the time necessary for a vehicle to travel some known length of a roadway. For these reasons, it is also signified simply as u. Space mean speed gives more emphasis to high ui, for this reason, us ut. The special value of speed is free to flow speed FFS or uf which defines the spaceman speed on the particular part of the road which is reached by an unrestricted traffic flow under prevailing roadway conditions HCM2010 defines. FFS as the mean speed of passenger cars operating inflow less than 1 000 pc hr ln FFS is Introduction. 11 determined by road geometry cross-section quality of road surface and all psychological factors making an impact on drivers.
Density. Density is defined as the number of vehicles per unit length of the roadway at a time instant. The standard notation is k and its unit is vehicles per kilometre veh km or vehicles per kilometer per lane veh km. ln It is expressed as k n D where n is the number of vehicles occupying length. D of the roadway at some specified time. It is rather difficult to measure density directly unless there is an opportunity to use aerial or satellite photography. Therefore it is more often estimated indirectly by measuring the inflow and outflow of vehicles at a road section over time but the initial state must be known in that case. The special case of density is so-called jam density kj which is the maximum possible density on the roadway when the speed of the flow is nearly zero. Relationships among traffic characteristics. If there is a requirement of analysis of traffic conditions the macroscopic approach is used. The fundamental equation describing average conditions on a given link for a specific time period is q u k. The equation assumes that the flow is uninterrupted and stable i.e all traveling at about the same speed. The fundamental diagram in the presented form assumes a linear speed-density model. That assumption is represented by Greenshield's traffic stream model. However, it is not the only traffic stream model that can be used. Even their combinations creating multi regime models might be applied. The benefit of using a linear representation of the speed-density relationship is that it provides a basic insight into the relationships among traffic flow speed and density interactions without clouding these insights by the additional complexity that a nonlinear speed-density relationship introduces. However, it is important to note that field studies have shown that the speed-density relationship u f k tends to be nonlinear at low densities and high densities. In fact the overall speed-density relationship is better represented by three relationships: 1 a nonlinear relationship at low densities that has speed slowly declining from free flow value uf, 2 a linear relationship over the large medium-density region and 3 a nonlinear relationship near jam density kj as the speed asymptotically approaches zero with increasing density Mannering Washburn.
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