Highway & Transportation (I) ECIV 4333 Chapter (4): Traffic Engineering Studies. Spot Speed

Transcription

1 Introduction Spot Speed Speed is an important measure of the quality of travel and safety of road network. Speed by definition is the rate of movement of vehicle in distance per unit time. A typical unit of speed is kilometers per hour (kph) or miles per hour (mph). Basically, there are two types of speed: the time-mean speed and the space-mean speed. Space-mean speed is the length of a road section divided by the average travel time of several vehicles over this specific section. The time-mean speed (spot speed) is the average spot speed of several vehicles measured at a given spot. Theory and Concept The methods used for conducting spot speed studies can generally be divided into two main categories: manual and automatic. Several automatic devices that can be used to obtain the instantaneous speeds of vehicles at a location on a highway are now available on the market. These automatic devices can be grouped into three main categories: (1) using road detectors, (2) by radar-based. (3) using the principles of electronics. Data Collection and methodology 1- The data is normally based on randomly sampling individual vehicle speed over short time period. It depends on observing the time required by vehicle to cover a short distance of roadway. Requirements: Stop watch. Measuring tab. 1

2 A X B This method is not very accurate. Limitations: Number of vehicles on the sample 100 veh. Distance between the two specified points 100 m. Veh. No. 1 Time(s.) (t) Velocity (m/s) V=(x/t) Velocity (Km/hr) Plot the frequency and the cumulative frequency distribution of the speed data. 3- Calculate the: - Average Speed. - Median Speed. - Modal Speed. - The 85th and 15th percentile speed. - Pace Speed. - Standard Deviation. 4- Check whether the speed distribution is reasonably normal. 2

3 Calculation The calculated mean (or average) speed is used to represent the true mean value of all vehicle speeds at that location. The accuracy of this assumption depends on the number of vehicles in the sample. The larger the sample size, the greater the probability that the estimated mean is not significantly different from the true mean. It is therefore necessary to select a sample size that will give an estimated mean within acceptable error limits. Statistical procedures are used to determine this minimum sample size. Before discussing these procedures, it is first necessary to define certain significant values that are needed to describe speed characteristics. They are: 1. Average Speed which is the arithmetic mean of all observed vehicle speeds (which is the sum of all spot speeds divided by the number of recorded speeds). It is given as where u = arithmetic mean Ii = number of observations in each speed group Uj = mid value for the ith speed group N = number of observed values. 2. Median Speed which is the speed at the middle value in a series of spot speeds that are arranged in ascending order. 50 percent of the speed values will be greater than the median; 50 percent will be less than the median. 3. Modal Speed which is the speed value that occurs most frequently in a sample of spot speeds. 4. The ith-percentile Spot Speed which is the spot speed value below which i percent of the vehicles travel; for example, 85th-percentile spot speed is the speed below which 85 percent of the vehicles travel and above which 15 percent of the vehicles travel. 5. Pace Speed: the highest speed within a specific range of speeds that represents more vehicles than in any other like range of speed. The range of speeds typically used is 10 km/h or 10 mph. Pace The 10 Km\h speed range representing the speeds of the largest percentage of vehicles in the traffic stream. 6. Standard Deviation of Speeds which is a measure of the spread of the individual speeds. It is estimated as 3

4 where S = standard deviation u = arithmetic mean Uj = jth observation N = number of observations 4

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