INTRODUCTION TO TRAFFIC ENGINEERING 1.1 Definition, scope and goal Definition of Traffic Engineering --- It is the phase of transportation engineering that deals of roads, streets with: and highways, planning
their networks and geometric design andterminals, traffic operations abutting lands, and relationships with other modes of transportation Transportation Engineering is defined as a discipline applying technology and scientific principles to the planning, functional design, operation, and
management of facilities for all modes of transportation Scope of Traffic Engineering --- surface (land) transportation; relationships and connection with other modes of transportation Major modes of surface transportation --automobile, bus, truck and bike Goal of Traffic Engineering --- explore how to provide for the safe, rapid,
comfortable, convenient, economical, and environmentally compatible movement of people and goods.
Safe --- public safety Rapid --- time value and customer service Comfortable/convenient --- level of service Economical --- social cost Environmental --- clean air and sustainability Movement = mobility Basic Elements of Highway Traffic Analysis In order to measure its level of effectiveness, certain parameters
associated with the highway must be measured and analysed. These properties include: The quantity of traffic The type of vehicles within the traffic stream The distribution of flow over a period of time (usually 24 hours) The average speed of the traffic stream The density of the traffic flow. Analysis of these parameters will directly influence the scale and layout of the Speed, flow and density of a
stream of traffic Where, q= traffic flow n=no. of vehicle l=a given length of roadway k= density or In general terms, q is expressed in concentration of vehicles per unit time. traffic where the traffic density, k, is a measure of
the number of vehicles, n, occupying a length of roadway, l. It can be seen that if the expression for q is divided by the expression for k, the expression for u is obtained: thus, the three parameters u, k and q are directly related under stable traffic conditions: This constitutes the basic relationship between traffic flow, space mean speed and density.
Speed-density relationship In a situation where only one car is travelling along a stretch of highway, densities (in vehicles per kilo-metre) will by definition be near to zero and the speed at which the car can be driven is determined solely by the geometric design and layout of the road; such a speed is termed free-flow speed as it is in no way hindered by the presence of other vehicles on the highway. As more vehicles use the section of highway, the density of the flow will increase and their
speed will decrease from their maximum free-flow value (uf) as they are increasingly more inhibited by the driving manoeuvres of others. If traffic volumes continue to increase, a point is reached where traffic will be brought to a stop, thus speeds will equal Greenshields (1934) proposed the simplest representation between the two variables, assuming a linear relationship between the two (see Fig. Below) Thus, the limiting values of the relationship between speed and
density are as follows: When k=0,u=uf and, When u =0,k=kj In mathematical terms, this linear relationship gives rise to the following equation: This assumption of linearity allows a direct mathematical linkage to be formed between the speed, flow and density of a stream of traffic. Flow-density relationship
From the equation derived above, we have: This is a parabolic relationship and is illustrated in the Figure In order to establish the density at which maximum flow occurs, is differentiated and set equal to
zero as follows: the term within the brackets must equal zero, therefore: km, the density at maximum flow, is thus equal to half the jam density, kj. Its location is shown (see the slide above) Speed-flow relationship In order to derive this relationship, rearrange the equation to
and Combining it with You will get This relationship is again parabolic in nature. In order to find the speed at maximum flow, is differentiated and put equal to zero: the term within the brackets must equal zero, therefore: um, the speed at maximum flow, is thus equal
to half the free-flow speed, uf. Its location is shown in Fig. (see the above slide) Combining Equations: the following expression for maximum flow is derived: Capacity and level of service analyses The no. of vehicles on our highways increases every year ,and transportation
engineers are often faced with the challenge of designing modifications to existing facilities that will service the increased demand. Capacity: is the maximum no. of vehicles that a given highway can accommodate. LOS: is an operating condition under capacity. Traffic engineers use capacity & LOS analysis to: o Determine the no. and width of lanes
needed o Assess service levels and operational characteristics of existing facilities. o Identify traffic and roadway changes Levels of Service LOS concepts Highway capacity manual defines the LOS categories for freeways and multilane highways as follows: LOS A Free-flow operation Reasonably free flow
Ability to maneuver is only slightly restricted Effects of minor incidents still easily absorbed From Highway Capacity Manual, 2000 LOS B Levels of Service LOS C Speeds at or near FFS Freedom to maneuver is
noticeably restricted Queues may form behind any significant blockage. Speeds decline slightly with increasing flows Density increases more quickly Freedom to maneuver is more noticeably limited Minor incidents create
From Highway Capacity Manual, 2000 LOS D Levels of Service LOS E Operation near or at capacity No usable gaps in the traffic LOS F Breakdown in flow Queues form behind breakdown points
Demand > capacity From Highway Capacity Manual, 2000 stream Operations extremely volatile Any disruption causes queuing LOS determination Base condition: * lane width * lateral clearance stream
* access frequency characteristics * terrain * traffic condition * driver population Steps: 1) Measure Free-flow speed from the field-is the mean speed of traffic as measured when flow rates are low to moderate 2) Analysis flow rate the highest volume in a 24-hr (the peak-hr volume) is used for V in traffic
analysis computation. 3) Determine LOS For further knowledge read Highway Engineering by Rogers M. THANK YOU QUESTIONS????
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