Traffic Engineering Studies(Spot Speed Studies)

Chapter 4

Dr. TALEB AL-ROUSAN

Introduction Availability of good transportation provided high

standard of living. However, several problems related to transportation

mode exist: Highway related crashes. Parking difficulties. Congestion. Delay.

To reduce negative impacts of highway, it is necessary to collect information that describes the extent of the problem and identifies their location.

Such information is usually collected by conducting traffic surveys and studies.

Categories of Traffic Studies1. Inventories: provide a list of graphic display

of existing information (street widths, transit routes, traffic regulations, available parking spaces, etc…).

2. Administrative studies: use existing engineering records to prepare an inventory of the relevant data which might include also the results of surveys.

3. Dynamic studies: Involves the collection of data under operational conditions and includes studies of speed, traffic volume, travel time and delay, parking, and crashes

Dynamic Studies/ Spot Speed Studies

Spot speed studies are conducted to estimate the distribution of speeds of vehicles in a stream of traffic at a particular location on a highway.

Speed: the rate of movement of a vehicle (mi/h) or (km/h).

Carried out by recording the speeds of samples of vehicles at specified location.

Speed characteristics will be valid only for the traffic and of environment conditions that exist at the time of the study.

Speed Characteristics Uses Establish parameters for traffic operations and control

(e.g. speed zones, speed limits (85th%), and passing restrictions.

Evaluate the effectiveness of traffic control devices (e.g. variable message signs at work zones).

Monitor effects of speed monitoring programs. Evaluate or determine the adequacy of highway

geometric characteristics (e.g. radii of H. curves and length of V. curves).

Evaluate the effect of speed on highway safety through analysis of crash data for different speed characteristics.

Determine weather complaints about speeding are valid.

Determine speed trends.

Location for Spot Speed Studies Depends on the anticipated use of the results and include:

1. Locations that represent different traffic conditions on highways: used for basic data collection.

2. Midblocks for urban highways and straight level sections of rural highways: used for speed trend analysis.

3. Any location can be used for the solution of a specific traffic engineering problem.

Unbiased data should be obtain which require that: Drivers should be unaware of the study being conducted. Equipments (radars) are concealed fro drivers. Observers conducting the study should be inconspicuous. Statistically adequate number of vehicle speeds be

recorded.

Time of Day and Duration of Spot Speed Studies

Time of day for conducting a speed study depends on the purpose of the study.

When purpose is to establish posted speed limits, observe speed trends, or collect basic data: It is recommended to conduct the study when traffic is free-flowing (i.e. Off-peak hours).

When speed study is conducted in response to citizen complaints: It is useful if the time period selected reflect the nature of the complaints.

Duration of the study should be such that the minimum number of vehicle speeds required for statistical analysis is recorded.

Duration is typically at least 1 hour and the sample size is at least 30 vehicles.

Sample Size for Spot Speed Studies The calculated mean (Average) speed is used to

represent the true mean value of all vehicle speeds. The accuracy of this assumption depends on the

number of vehicles in the sample (larger sample size increase the probability that the estimated mean is not significantly different from the true mean.

Minimum sample size depends on the precision level desired.

Precision level: the degree of confidence that the sampling error of a produced estimate will fall within a desirable fixed range.

Confidence level is given in terms of (α) where α = (100 – confidence level).

Commonly used confidence level for speed counts is 95%.

Sample Size for Spot Speed Studies Cont.

It is also assumed that the normal distribution describes the speed distribution.

The properties of the normal dist. Are then used to determine the min. sample size of an acceptable error (d) of the estimated speed.

N= [(Z /d]2

N: min. sample size.Z: number of standard deviations corresponding to the

required confidence (1.96 for 95% confidence… See Table 4.1)

: Standard deviation (mi/h)d: limit of acceptable error in the average speed

estimate.

Normal Distribution Given by:

f(x) =(1 sqrt(2))) e^ [–(x-)2/22]= true mean of population and = true standard deviation Basic Properties:

Symmetrical about the mean Total area under curve =1.0 or 100%. Area under curve between () and () = .6827 Area under curve between () and ()

= .9545 Area under curve between () and () = .9545 Area under curve between () and () = .9971 Area under curve between (m+ ∞s) and (m- ∞s) = 1.0

Normal Distribution Specific conclusions can be drawn

from these properties: For example: 95% of all vehicle speeds

will be between () and ().

Similarly , if a vehicle is selected at random, there is a 95% chance that its speed will be between () and ().

Table 4.1 Constant Corresponding to Level of Confidence

Confidence Level %Constant (Z)

68. 31.00

86.61.50

90.01.64

95.01.96

95.52.00

98.82.50

99.02.58

99.73.00

Significant Values That Describe Speed Characteristics

1. Average Speed: Arithmetic mean of all observed vehicle speeds at that location (Sum of all spot speeds divided by the number of recorded speeds).

ū = [∑(fi ui)]/ ∑(fi)Ū= Arithmetic meanfi =number of observations in each speed group

ui = mid value for the ith speed groupor

ū = [∑(ui)]/ NN=Number of observed valuesui =speed of the ith vehicle

Significant Values That Describe Speed Characteristics Cont.

2. Median Speed: the speed at the middle value in a series of spot speeds that are arranged in ascending order. 50% of the speed values will be greater than the median and 50% will be less than the median.

3. Modal Speed: The speed value that occur most frequently in a sample of spot speeds.

4. The ith percentile spot speed: The speed value below which i percent of the vehicles travel.

5. Pace: The range of speed (usually 10 mi/h interval) that has the greatest number of observations.

Significant Values That Describe Speed Characteristics Cont.

6. Standard deviation of speeds: A measure of the spread of the individual speeds, and is given by

S = Sqrt [∑(ui- ū)2]/(N-1)]

Or when presented as classes

S = Sqrt [[∑(fi ui2) - (∑(fi ui))2 /∑(fi)]/ ∑(fi-1)]

S = Sqrt [∑(fi (ui- ū)2)/ (N-1)]

fi : frequency of speed class iui :mid value of speed class i

Methods of Conducting Spot Speed Studies

Two Methods available:1. Manual: Seldom used2. Automatic: several automatic devices

available to obtain instantaneous speed, which may be grouped into 3 categories:

1. Road detectors.2. Doppler principle meters (Radars).3. Devices that use principles of electronics.

Road Detectors Two groups:

Pneumatic road tubes. Induction loops.

These detectors can be used to collect data on speeds at the same time the volume data are being collected.

They are laid with 3 to 15 ft distance between detectors

Advantages: human errors are reduced. Disadvantages:

Expensive When pneumatic tubes are used, they are

conspicuous, which affect the driver behavior resulting in distortion of speed distribution.

Road Detectors/ Pneumatic Road Tubes

Laid across the lane. When a vehicle passes over the tube, an air impulse is

transmitted through the tube to the counter. When used for speed measurements, two tubes are

placed across the lane about 6 ft apart. Impulse recorded when front wheels passes over the

first tube Shortly afterward a second impulse is recorded when

the front wheel of the same car passes over the second tube.

The time elapsed between the two impulses and the distance between the tubes are used to compute speed of the vehicle.

Road Detectors/ Inductive Loops

Rectangular wire loop buried under the roadway surface.

It serves as the detector of a resonant circuit.

It operates on the principle that a disturbance in the electrical field is created when a motor vehicle passes a cross it, Which cause a change in potential that is amplified, resulting in an impulse being sent to the counter.

Doppler-Principle Meters (Radars) Works on the principle that when a signal is

transmitted onto a moving vehicle, the change in frequency between the transmitted signal and the reflected signal is proportional to the speed of the moving vehicle.

Advantages: if equipment is located in inconspicuous position, the influence on driver behavior will be considerably reduced.

Examples: SpeedAce Meter: pocket-sized, hand-held laser speed

detection. Used to measure speed of individual vehicles at a range of up to 1312 ft.

RTMS meter: multilane presence radar. Can be mounted on the side of the highway and obtain data on speeds of vehicles in up to 8 lanes seperately.

Electronic-Principle Detectors Presence of vehicle is detected through

electronic means, and information on these vehicles is obtained.

Advantage over road detectors is that it is not necessary to physically install loops or detectors on the road.

Example: Video image processing (known as machine-vision-system).

Consist of : electronic camera + microprocessor which determine the traffic characteristics in real time.

Electronic-Principle Detectors Example on such system is Autoscope. Autoscope developed in USA Autoscope: a wireless detector with a single camera

that can replace many loops, thereby providing a wide area detection system.

Advantage: Monitor many locations within the camera field of

view.. location can be selected by user. Can be installed without disturbing traffic operations. Can extract traffic parameters like volume and queue

length.

Presentation & Analysis of Spot Speed Data

The data collected from a sample of vehicles are used to determine the speed characteristics of the whole population of vehicles traveling on the study site.

Statistical methods are used for analysis. Several characteristics can be determined

either by direct calculations or by graphical presentation.

Presentation format most commonly used is the frequency distribution.

Presentation & Analysis of Spot Speed Data Cont.

Steps to prepare frequency dist. Table:1. Select number of classes (# of velocity ranges): usually

between 8 – 20 (Another technique: determine range for a class size of 8, then determine range for a class size of 20, by dividing the difference between max and min speeds by 8 then by 20, then selecting a range between these max and min ranges).

2. The mid value for each class is used as a speed value for that class

3. Plot frequency histogram (speed mid values vs frequency)4. Frequency distribution curve (speed mid values vs percentage

of frequency in each class).5. Cumulative distribution (upper limits of speed classes vs

cumulative percentage of frequency) See Example 4.2 for determining the speed

characteristics from a set of speed data.

Comparison of Mean Speeds It some times necessary to determine

whether there is a significant difference between the mean speeds of two spot speed studies.

This is done by comparing the absolute difference between the sample mean speeds against the product of the standard deviation of the difference in means and the factor Z for a given confidence level.

If the absolute difference is greater, it can be then concluded that there is a significant difference in sample means at that specific confidence level.

Comparison of Mean Speeds Cont. The standard deviation of the difference in means is

given by:Sd = Sqrt[(S1

2/n1) + (S22/n2)]

ni = sample size of study iSi

2= variance about the mean of study I If u ū1 mean speed of study 1, ū2 mean speed of study 2,

and

l ū1 – ū2l > Z Sd it can be concluded that the mean speeds are

significantly different at the confidence level corresponding to Z.

Since it is usual to use the 95% confidence level in traffic engineering studies, the conclusion therefore will be based on whether l ū1 – ū2l is greater than 1.96 Sd

7ٍSee Example 4.3.