lecture 2-speed studies

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Traffic Engineering Dr. Aso Faiz Talabany

Traffic EngineeringLecture No. 2

SPEED STUDIES

Speed is an important transportation considerationbecause it relates to safety, time, comfort, convenience,and economics.

Definitions:

Speed: is a rate of motion (movement of vehicle)expressed as distance per unit time, generally as(kph) or (mph).

Spot speed: is the instantaneous speed of a vehicle at aspecified location of road.

Average spot speed: The arithmetic mean of the speedsof all traffic at a specified point.

Overall travel time: The time of travel including stopsand delays (except those off the traveledway).

Running time: The time the vehicle is in motion.

Overall travel (journey) speed: The distance divided bythe overall travel time.

Running speed: The distance divided by the runningtime.

Design speed: Is the maximum safe speed that can bemaintained over a specified section of highway(a speed determined for design).

Operating speed: The highest overall speed, exclusive ofstops, at which a driver can travel on a

given highway under prevailing conditions,

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without at any time exceeding the designspeed.

Space mean speed (SMS or Vs): The speed

corresponding to the average travel timeover a given distance (it is the harmonicmean).

Time mean speed (TMS or Vt):The speed correspondsto the average of overall travel speeds andit is higher than the SMS for the samesample (it is the arithmetic mean).

Use of Speed Studies

Speed measurements are needed for a number ofpurposes given below:

Spot speeds are used for:1- Geometric design of roads2- Regulation and control of traffic operations3- Analyzing the causes of accidents4- Before and after studies of road improvements

5- Determining the problems of congestion onroads6- Trends in operating speeds of different vehicles7- Speeds at problem locations8- Research studies

Spot speed have a number of safety applications,including the following (Robertson 1994):

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Travel (journey) speeds:1- The cost of the journey

2- To evaluate congestion, capacity, LOS, and theneed for improvements3- Trip assignment in transportation planningstudies

Spot Speed StudiesPurpose:

The vehicle spot speed study is designed to measurethe speed characteristics at a specified location under thetraffic and environmental conditions prevailing at the timeof the study.

Factors which affect the spot speed1- The driver2- The vehicle3- The roadway4- The traffic5- The environment

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watch method is a quick and inexpensive method forcollecting speed data.

Pneumatic road tube methodThe pneumatic road tube method is normally used for

longer data collection time periods than those of either thestopwatch or radar meter method. Using this method,pneumatic tubes are placed in the travel lanes and areconnected to recorders located at the side of the road.

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2- Radar meter methodA radar meter is commonly used device for directly

measuring speed studies. This device may be hand held, mounted on a tripod. The effective measuring

distance for radar meters ranges from 200 feet up to 2miles. A radar meter requires line of sight toaccurately measure speed and is easily operated by oneperson. If traffic is heavy or the sampling strategy iscomplex, two radar units may be needed.

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lecture 2-speed studies

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