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Transport Modelling

Syllabus Traffic flow theory Macroscopic flow models Microscopic flow models VISSIM project. Transport Modelling. Traffic Stream Characteristics.

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Transport Modelling

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  1. Syllabus • Traffic flow theory • Macroscopic flow models • Microscopic flow models • VISSIM project Transport Modelling

  2. Traffic Stream Characteristics • Traffic streams are not uniform, but vary over both space and time. Because of that, measurement of the variables of interest for traffic flow theory is in fact the sampling of a random variable. • In reality, the traffic characteristics that are labelled as flow, speed, and concentration are parameters of statistical distributions, not absolute numbers. • Traffic Flow Variables of Interest: • Rates of flow (vehicles per unit time); • Speeds (distance per unit time); • Travel time over a known length of road • Occupancy (% of time a point on the road is occupied by vehicles); • Density (vehicles per unit distance); • Time headway between vehicles (time per vehicle); • Spacing, or space headway between vehicles (distance per vehicle); • Concentration (measured by density or occupancy)

  3. Measurement Procedures • measurement at a point • measurement over a short section • measurement over a length of road • the use of an observer moving in the traffic stream • wide-area samples obtained simultaneously from a number of • vehicles as part of Intelligent Transportation Systems (ITS)

  4. Examples of different measurement techniques • Point Method: Flow rate • Short section: Speed, Occupancy • Length of roadway: Density, average flow and average speed • Moving Observer: Floating car procedure in which speeds and travel times are recorded as a function of time and location along the road. While the intention in this method is that the floating car behaves as an average vehicle within the traffic stream, the method cannot give precise average speed data. It is, however, effective for obtaining qualitative information about freeway or highway operations without the need for elaborate equipment or procedures. • ITS wide-area measurement: ITS involve the use of communications from specially-equipped vehicles to a central system. The system provide for transmission of information on the vehicles' speeds. In some cases, it is the instantaneous speed while passing a particular reporting point. In others, the information will be a vehicle identifier, which would allow the system to calculate journey times between one receiving location and the next. A third type of system will use poll vehicles regardless of location, and would receive speeds and location information back from the vehicles.

  5. Flow Rate or Traffic Flow • Flow (q) or volume is the number of vehicles passing a point on a road or a lane during a specified time period. OR, • Flow rate is the equivalent rate at which a vehicle passes a point on a road or a lane during unit time interval • Flow rates are usually expressed in terms of vehicles per hour, although the actual measurement interval can be much less. Concern has been expressed, however, about the sustainability of high volumes measured over very short intervals (such as 30 seconds or one minute) when investigating high rates of flow. The 1985 Highway Capacity Manual (HCM 1985) suggests using at least 15-minute intervals, although there are also situations in which the detail provided by five minute or one is termed the time mean speed, because it is an average of minute data is valuable. • In this context we will discuss about different types of measured traffic flow or volumes.

  6. Traffic Volume • Traffic flow vary over time and is normally expressed as volume with respect to the duration of measurement (vehicles per unit time) • Daily Volume • Annual Average Daily Traffic (AADT) • Average Daily Traffic (ADT) • Average Weekly Traffic (AWT) • Hourly Volume • Peak Hourly Volume (PHV) • Design Hour Volume

  7. Daily Traffic Volume Measures • Mainly used for transport planning • The unit is vehicles per day • AADT: The average daily traffic volume at a given location over a year- i.e., the total number of vehicles passing the site in a year divided by 365 • ADT: Average daily traffic volume at a given location for some period of time less than a year. • AWT: Average daily traffic volume occurring on weekdays for some period less than a year, when averaged over a year then called AAWT

  8. Hourly Traffic Volume Measures • Used for traffic control as well as planning purposes • The unit is vehicles per hour • PHV: the traffic volume measured over the busiest hour of the day at a given location. PHV gives the highest hourly volume in a day • PHF: Peak hour factor • For 15 min periods, • DHV: Peak hourly volumes are different for every day of the year. The thirtieth highest peak hour volume is considered for rural design and the fiftieth highest peak hour volume is considered for urban design and are often called DHV • DHV = AADT X k • where, k is the ratio of 30th or 50th HV to the AADT from a similar site

  9. Types of Vehicle Counts & Usage

  10. Speed • Speed is the distance traversed by a vehicle in unit time, expressed in terms of kilometers per hour (kmph) or miles per hour (mph) or meter per second (m/s) • Defined as the inverse of the time taken by a vehicle to traverse a given distance. It is an important measure of the quality of traffic operation. Drivers can directly perceive this quantity. • Measurement of the speed of an individual vehicle requires observation over both time and space. The instantaneous speed of an individual vehicle is defined as • In a traffic stream, speed of different vehicles need not be the same at a given time and location. Therefore speed of a traffic stream is not a single value, but is a distribution of individual vehicle speeds. • Different types of average values of speed are used to characterize a traffic stream. • time mean speed • space mean speed

  11. Space mean speed (SMS)Time Mean Speed (TMS) • The average speed of vehicles measured at a point/location over a given interval of time. TMS is the arithmetic mean of all speeds • SMS is the average speed of vehicles measured at an instant of time over a specified stretch of road. SMS is the harmonic mean of all speeds. • E.G. • v1=12 m/s, v2 =15 m/s and v3= 10 m/s

  12. Speed Calculation

  13. Relationship between TMS and SMS

  14. Relationship between TMS and SMS

  15. Other Speed Measures • Average Running Speed: It is a type of SMS. Defined as the average speed of a vehicle in motion on a large stretch of road • Average Travel Speed: It is a type of SMS. Average speed of a vehicle on a large stretch of road including the stopped delay. • Free Flow Speed: The desired speed of a vehicle under ‘no congestion conditions’ or very low volume conditions • Percentile Speed: A speed below which the stated percent of vehicles in the traffic stream will travel.

  16. Density, Occupancy • Density is the number of vehicles occupying a given length of a lane or a roadway at a particular instant expressed as vehicles per kilometer (vpkm) or vehicles per kilometer per lane (vpkmpl) • It is a measure directly related to traffic demand and • It is a measure of the quality of traffic operation and driver’s behavior significantly depends on density

  17. Density, Occupancy • Density and occupancy both of are considered under the broad term ‘concentration’ • Occupancy is the fraction of time that vehicles are over the detector. For a specific time interval, T, it is the sum of the time that vehicles cover the detector, divided by T. For each individual vehicle, the time spent over the detector is determined by the vehicle's speed, its length, the length of the detector itself. That is, the detector is affected by the vehicle from the time the front bumper crosses the start of the detection zone until the time the rear bumper clears the end of the detection zone.

  18. Decide on a network (min. 4 junctions) • Model the existing scenario • Propose a change & model the changed scenario • Prepare a report comparing the modelled scenarios • Content of the Report • VISSIM file with current network • VISSIM file with hypothetical network • Changes in travel time, traffic volume, speed, delay and queuing due to the proposed change • A qualitative discussion on the suggested change VISSIM Project

  19. Project Assignment on 23rd November • Things you need to bring and things that will be discussed in the lecture: • The network that you are going to model (Bring a print out of the Google map of the network or something similar) • Discuss the current problems/issues with the network • Discuss your proposed changes • What improvement do you expect to see if your proposed changes are taken effect

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