Traffic Volume

Traffic volume or rate of Flow (Q) is defined as the number of vehicles passing

a point on a highway, or a given lane or direction of a highway, during a specified time interval.

“Vehicles per unit time.” No of Vehicle/hr (or) No of Veh/day

Q=K*V

-It is done by manual or automatic counters. In this study, the type of vehicles, Number of vehicles and

Direction of vehicles to be counted.

- In view of variety of vehicles a Number is assigned to a vehicle. Based on SPEED, SPACE and CHARACTERISTICS. In comparison to the car termed as equivalent passenger car unit (PCU). -PCE (Passenger Car Equivalent): The impact that a mode of transport has on traffic variables (speed, density and headway) compared to a single car.

-PCU (Passenger Car Unit): It is a vehicle

unit used for expressing highway capacity unit. Used to convert different vehicles to one class.

Methods of Volume Counting: 2

1. Manual Counts: are necessary when automatic equipment is not available. Manual counts are typically used for periods of less than a day. Normal intervals for a manual count are 5, 10 or 15 min’s. as per IRC, for rural roads traffic count shall be taken over seven consecutive days and 24 hours during each day. Counts are taken at least twice each year. Once count

Traffic Speed

Speed (V) is defined as a rate of motion in distance per unit time.

Travel time is the time taken to traverse a defined section of

roadway.

Speed = Distance/Time(m/sec) V=Q/K

-In a moving traffic stream, each vehicle travels at a different speed. -Thus, the traffic stream does not have a single characteristic value, but rather a distribution of individual speeds. -The traffic stream, taken as a whole, can be characterized using an average or typical speed.

Types of Speeds: 3

a. Spot speed: instantaneous speed of a vehicle at a specified location. b. Running speed: average speed maintained over a given route while vehicle in motion. c. Overall, travel or journey speed: effective speed in a given route a.Methods to determine SPOT Speeds:

-The methods available for measuring spot speed can be grouped as those observations that require time taken by a vehicle to cover a known distance. -It consists of long base methods- vehicles are timed over a long distance. -Short base methods- vehicles are timed over a Short distance say about 2m. I). those observations that require time taken by a vehicle to cover a known distance.

Traffic Density

Density (K) is defined as the number of vehicles occupying a given length of

highway or lane.

Vehicles per mile (or) vehicles

per mile per lane. K=Q/V

-Density is difficult to measure directly, as an elevated vantage point from which the highway section under study may be observed is required. -Density is difficult to measure directly; modern detectors can measure occupancy, which is a related parameter. -Occupancy is defined as the proportion of time that a detector is "occupied," or covered, by a vehicle in a defined time period.

If “occupancy” over a given detector is

“0,”then density

Lv is the average length of a vehicle (ft) Ld is the length of the detector (which is normally a magnetic loop detector). -It is often computed from speed and flow rate measurements. -Density is also an important measure of

the quality of traffic flow, as it is a measure of the proximity of other vehicles, a factor which influences freedom to manoeuvre and the psychological comfort of drivers.

Traffic characteristics and Traffic measurement by PENKI RAMU is licensed under a Creative Commons Attribution 4.0 International License.

being taken during the peak season of harvesting and marketing and other lean season. Traffic counts during a Monday morning rush hour and a Friday evening rush hour may show exceptionally high volumes and are not normally used in analysis; therefore, counts are usually conducted on a Tuesday, Wednesday, or Thursday. Procedure -No. of observers required depends upon no. of Lanes and type of information required. -More desirable to record traffic in each direction of travel by posting separate observers for each direction. -For all-day counts, work in three shifts can be arranged. -Data is recorded by 5 dash system. (IIII) Example: Tally Sheets: Recording data onto tally sheets is the simplest means of conducting manual counts. The data can be recorded with a tick mark on a pre-prepared field form. A watch or stopwatch is necessary to measure the desired count interval. ADVANTAGES: -Details such as vehicle classification and no. of occupants can be obtained. -Records Turning movement data of vehicles at intersections. -Analyzing traffic characteristics in unusual conditions such as adverse weather conditions, traffic breakdowns, temporary Closure of any lane of highway. -Comparatively cheaper method of counting and Data accumulated by manual methods are easy to analyze. Disadvantages: Not practicable to have manual count for 24 hrs of the day and throughout the year.

a)Direct timing Procedure: Simplest method, Two reference points are marked on the pavement at a suitable distance apart and an observer starts and stops stopwatch as a vehicle crosses these two marks. From known distance and time, speed can be calculated

.-Observation by two observers one at each reference points.

-First observer standing at the point in which the vehicle pass first, signals and seconds observer starts the stop watch. -The second observer stops the stop watch when he observes the same vehicle passing the reference point. Disadvantage: reaction time of two observers. b). Enoscope: Also known as mirror box

-L-shaped box, open at both ends, with a mirror set at 45 ˚ to the arms of the instrument.

Spot speed Types: 2

1. Time mean speed: avg speed of all the vehicles passing a point on a highway over some specified Time period. Avg. of spot speed. (V1+v2+…+Vn)/N

2. Space Mean speed: avg. speed of all the vehicles occupying a given section of road over some specified time period. Harmonic

mean of spot speed. V*N

b. Model Speed: the speed at which greatest numbers of vehicles travel. It is the peak value of frequency distribution curve.

From fig model speed=50kmph C. Median speed: the middle or 50th percentile speed Presentation of Spot Speeds:

1. 85th %speed: the speed at or below which 85% of vehicles are passing a point on the highway or only 15% of vehicles exceed the speed at that point. This is adopted for safe speed limit at this zone. 2.98th % speed: for the speed purpose of the highway. 3. 15th % speed: the speed value which is used as minimum speed on major highways.

Prepared by P.Ramu Asst Prof. VNR VJIET

2. Automatic or Mechanical Methods: An automatic survey involves placing a tube or loop across a road which is connected to a box containing the means for storing the information. In this method, vehicles are counted automatically without any human involvement. There are two techniques of automatic counting: a) Contact system based on pneumatic, mechanical, magnetic or b) Contactless system based on electrical/optical, ultrasound/infrared radar, micro wave, CCTV/video image processing method etc. Suitability: for long counts and for permanent installations. Limitations: do not give directional counts or classified counts. So, the data is not as accurate compared to manual counting, since 2 or more vehicles going abreast will be recorded as single unit. Presentation of Traffic volume data: i. Annual Average Daily Traffic (AADT):

The total yearly volume divided by number of days in the year. ii. Average Daily Traffic (ADT): The total volume during a given time period, in whole days greater than one day and less than one year, divided by number of days in that time period. iii. Trend charts: Shows the hourly, daily or monthly changes in volume through an area. Useful for planning future expansion, design and regulation. iv. Traffic Flow Maps: Show volume along various routes by using bands proportional to traffic volume carried. Thickness of lines represents traffic volume. v. 3othHighest Hourly Volume: hourly volume exceeds only 29 times in a year. Congestion only during 29 hrs in the year.

-The instrument is placed directly opposite to the first reference point and the observer stations himself at the second reference point. Stop watch is started as soon as the vehicle passes the first point and is stopped as soon as it passes the observer. -If two enoscope is used, the observer stations himself midway between two reference points. Advantages: -Eliminates parallax error. -Equipment is of low cost, simple and can set up easily Shot base methods:

a) RADAR SPEED METERS: Works on the Doppler principle. Speed of a moving body is proportional to change in frequency between the Radio wave transmitted to the moving body and the radio wave received back. Instrument directly measures speed. Instrument is setup near the edge of carriageway at a height of 1m. The speed meter is so kept that the angle between direction of travel of vehicle and axis of transmission of radio wave is as low as possible, say within 20˚. Speed of vehicles in both directions can be observed.

b) Photographic Method: Time-lapse camera photography can be used. -Photographs are taken at fixed intervals of time (say one second per frame) on a special camera. By projecting the film on a screen, the passage of any vehicle can be traced with reference to time. Images by video cameras can be also used. c) Pneumatic Tube wire method d) Time Lapse method e) Photo electronic meter f) Electronic meter method

Fundamental diagram of traffic flow or

Macroscopic parameters: 3

1.Flow(Q) and Density (K): 1. When the density is zero, flow will also be zero, since there are no vehicles on the road. 2. When the number of vehicles gradually increases the density as well as flow. 3. When more and more vehicles are added, it reaches a situation where vehicles can't

move. This is referred to as the jam density or the maximum density. 4. At jam density, flow will be zero because the vehicles are not moving. 5. There will be some density between zero density and jam density, when the flow is maximum. 2.Density(K) and Speed(v): 1. speed will be maximum, referred to as the free flow speed, and when the density is maximum, the speed will be zero. 2. Corresponding to the zero density, vehicles will be flowing with their desire speed, or free flow speed. 3. When the density is jam density, the speed of the vehicles becomes zero. 4. It is also possible to have non-linear relationships as shown by the dotted lines. 3Speed(v) and Flow(Q) 1. The flow is zero either because there is no vehicles or there are too many vehicles so that they cannot move. 2. At maximum flow, the speed will be in between zero and free flow speed. 3. The flow is zero either because there is no vehicles or there are too many vehicles so that they cannot move. 4. At maximum flow, the speed will be in between zero and free flow speed. -can be represented with the help of some curves.

Traffic characteristics and Traffic measurement by PENKI RAMU is licensed under a Creative Commons Attribution 4.0 International License.