Traffic Studies- Traffic Volume Study & Speed Study
The traffic surveys for collecting traffic data are also called traffic census.
These Studies help in deciding the geometric design feature and traffic control for safe and efficient traffic movements.
The various traffic studies generally carry out are:
- Traffic Volume Study
- Speed Study
- Origin and Destination study
- Traffic Flow Characteristics
- Traffic capacity study
- Parking study
- Accident Studies
Traffic Volume Study
Traffic volume is the number of vehicles crossing a section of road per unit time.
It is used to measure the traffic flow (vehicle/day or vehicles/hour).
It can be determined by-
q= (n*3600/T) vph
n= the number of vehicles passing a point in the roadway in T sec.
q= the equivalent hourly flow
Complete traffic volume study includes:
- Classified volume study
- Directional study
Following are the means of conducting traffic surveys: By toll plaza ticketing, Registration offices, Statistical approach, By interview, By check post, Modern Global positioning studies.
Use and objectives of traffic volume studies
- It provides a true measure of the relative importance of roads and decides the priority of improvement and expansions.
- It is used in planning of traffic operations, control of existing facilities and designing of new facilities.
- It helps in analysis of traffic patterns and trends.
- It provides useful data for structural and geometric design of pavements and computation of road capacity.
- It is used in planning one-way streets and other regulatory measures.
- Turning movement study is used in the design of interactions, in planning signal timings, channelization, and other control devices.
- Pedestrian traffic volume study is used for planning side-walks, cross walks subways and pedestrian signals.
Traffic volume Calculation
- The traffic flow varies from time to time as hourly traffic volume varies considerably during day; hourly volume will be much higher than the average hourly volume.
- Daily traffic volumes vary in a week and during seasons.
- Therefore, true picture is to be obtained for patterns of hourly, weekly, daily, and seasonal variation including class of traffic (buses, truck, cars, motorcycle, and etc.).
- During traffic volume study traffic variations and the direction of each class of traffic is recorded with the information of their turning movements.
- Traffic volume counts is done with the help of mechanical counters or manually, using pneumatic tube, multipen recorder.
- At first the fluctuations of traffic volume during the hours of day and the daily variations are observed.
- After that with the help of statistics the peak hourly traffic volumes and average daily traffic volumes are calculated.
Presentation of Traffic Volume Data
Annual average daily traffic (AADT)
It is the average 24-hour traffic volume at a given location over a full 365-days. Total traffic and classified traffic are calculated. It helps in deciding the relative importance of a route & road development. It includes seasonal variations also.
Average Daily Traffic (ADT)
It is the average 24-hour volume at a given location for some period of time less than a year. For this minimum of 7-days count is done to include the daily variation like on Saturday and Sunday.
Showing volume trends over period of years. These data are useful for planning future expansion, design, and regulation.
Variation charts showing hourly, daily and seasonal variations are also prepared. These help in deciding the facilities and regulation needed during peak traffic periods.
Traffic Flow Map Along the Route
The thickness of the lines representing the traffic volume to any desired scale.
Thirtieth highest hourly Volume
It is found from the plot between hourly volume and the number of hours in a year that the traffic volume is exceeded. For this all hourly volumes are arranged in decreasing order and order no. is given to each of them. The data at order no. 30 is the 30th highest volume.
The 30th highest hourly volume is the hourly volume that will be exceeded only 29 times in a year and all other hourly volumes of the year will be less than this value.
Periodic Volume Counts
Hourly expansion factor= (Total vol. for 24-hr/Volume for particular hour)
Daily expansion factor= (Avg. total vol. for a week/Avg. vol. for particular day)
Monthly expansion factor= (AADT/ADT for particular month)
Thus, if 24-hr count at a location is done and hourly volume is calculated, we can calculate the hourly expansion factor for each hour.
Speed of different vehicles vary with respect to time & space. To present these variations several types of speed can be defined. These are-
It is the instantaneous speed of a vehicle at a specified location or instant. Spot speed is needed to design
- Horizontal and vertical curve
- Location and size of signs
- Design of signals
- Accident analysis
Spot speed is measured using Enoscope, pressure contact tubes and loop deflector, and doppler radar.
It is the average of spot speed of all vehicles passing a point on the road.
There are two types of mean speeds or average speeds-
- Time mean speed
- Space mean speed
Time mean speed (Vt)
Time mean speed is the arithmetic mean of the speed of the vehicle passing a point on a highway during an interval of time.
Vt= time mean speed (kmph)
Vi= observed instantaneous speed of ith vehicle (kmph)
n= number of vehicles observed
Space mean speed (Vs)
It is the average speed of vehicles over a certain road length at any time.
Space mean speed is the hormonic mean of the speed of the vehicles passing a point at a highway during an interval of time.
Vs= n/ (∑(1/ui)
Vs= Space mean speed
tavg= time taken by ith vehicles to cross the highway
ui= speed of the ith vehicle
Running Speed= length of travel/total time in which the vehicle was running
Running time excludes stopping delays.
Journey speed= length of travel/total journey
Total Journey time includes the stop delays
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