Traffic Engineering: Spot Speed Studies

Spot Speed Studies

• Conducted for a point (spot) or a short segment (<1,000 ft).
• Carried out to determine driver speed choices in free flow conditions.
• Not conducted when traffic volumes exceed 750 to 1,000 veh/h/ln on freeways or 500 veh/h/ln on other uninterrupted flow facilities.

Uses of Spot Speed Data

• Establishing the effectiveness of new or existing speed limits or enforcement practices.
• Determining appropriate sight distances and relationships between speed and highway alignment, and speed performance with respect to steepness and length of grades.
• Timing "yellow" and "all-red" intervals for traffic signals, and for placement of signs, and development of appropriate signal progressions.
• Investigating high-accident locations.

Speed Definitions of Interest

• Average or time mean speed: The average speed of all vehicles passing the study location during the study period (mi/h).
• Standard deviation: The average difference between individual observed speeds and the average speed during the study period.
• 85th percentile speed: The speed below which 85% of the observed vehicles travel (mi/h).
• Median speed: The speed that equally divides the distribution of spot speeds.
• Pace: A 10 mi/h increment in speeds that encompasses the highest proportion of observed speeds (compared to any other 10 mi/h increment).

Speed Data Collection

• Much speed data are collected using permanent detector locations, such as loop detectors.
• Or: using hand-held or vehicle-mounted radar "guns" or detectors.
• Individual observed speeds are arranged as frequencies of observations within defined speed groups.
• Typical breadth of speed group of 2 mi/hr; smaller ranges can be used with larger datasets
• More than 5 mi/hr should never be used.

Spot Speed Sample Data Analysis

• Typical breadth of speed group of 2 mi/hr; smaller ranges can be used with larger datasets.
• More than 5 mi/hr should never be used.

Frequency Distribution Table

• Data is presented as frequency counts, cumulative percentage, and related statistical analyses
• Data is summarized in table

Common Descriptive Statistics

• Central tendency: Measures that describe the approximate middle or center of the distribution.
• Dispersion: Measures that describe the extent to which data spread around the center of the distribution.

Central Tendency: Mean

• Provides a calculated average of values.
• Formula is Σ (nᵢ*sᵢ) / N

Dispersion: Standard Deviation

• In simplistic terms, the standard deviation of speeds is the average difference between observed speeds and the time mean speed during the study period for discrete values.
• In simplistic terms, the standard deviation of speeds is the average difference between observed speeds and the time mean speed during the study period for continuous data.

% of Vehicles Within Pace

• The smaller the percentage of vehicles traveling within the pace, the greater the degree of dispersion in the distribution.
• Over 60% of the vehicles traveled at speeds between 42.5 and 52.5 mi/h - standard deviation of approximately 5 mi/h.

Dispersion: 85th and 15th Percentile Speeds

• Summarize high and low speeds observed by most drivers.
• Closer values to the mean suggest less dispersion and stronger central tendency.
• Calculated from cumulative distribution curve at y-coordinate of 15 and 85 values.
• Expressed in mi/h.

Using the Normal Distribution in the Analysis of Spot Speed Data

• Most speed distributions are statistically normal.
• Standard notation x~N[40,25] signifies the variable 'x' is normally distributed with a mean of '40' and a variance of '25'.
• Standard normal distribution, where z~N[0,1].

Precision and Confidence Interval

• The standard deviation of a distribution of sample means with a constant group size of n
• E = s/√n
• Assuming a normal distribution, it is known that 95% of all values lie between the mean ± 1.96 standard deviations; 99.7% of all values lie between the mean ±3.00 standard deviations.
• A measure of how close the sample statistic is to the true population parameter.

Required Sample Size

• The formula expresses the necessary sample size for desired accuracy and confidence levels. This is useful for planning the study.
• Formula n= (3.8452 x s^2) / (e^2)

Sample Size Problem

• Determining the number of observations needed to estimate the true mean speed of an observed system to specified tolerance and confidence.

Before and After Spot Speed Studies

• The difference between two sample means follows a normal distribution if the samples are from the same population and if the sample size is 30 or more in each group.
• Two important questions include if the difference in mean speed is significant and if the desired reduction in speeds was achieved (using statistical tests).

Check for Normalcy: Chi-Square

• A statistical test to assess whether the data follows a normal distribution.
• Formula χ² = Σ (nᵢ - fᵢ)² / fᵢ, where N G = Number of speed groups in the distribution

Example

• Spot speed results (before and after a speed limit change) are presented to demonstrate application of the concepts introduced.

Speed Group

• Data is presented to show how speed groups (mph ranges) are defined and how counts are taken within each range.

Frequency Distribution Table (Page 7)

• Summarizes data into numerical frequency, the percentage of occurrences in each category

Travel Time Studies

• Similar to journey time surveys (JTS), they involve measuring travel times over significant lengths of highway segments/routes, often in conjunction with delay studies.
• Data collection techniques vary.
• Applications include identifying problem locations, measuring arterial level of service (LOS), and input for traffic assignment models.
• Data collection techniques include test cars, license plates, and observers.

Field Study Techniques - Test Car

• Test cars are equipped to record times and causes of delays.
• Floating car method: The driver strives to maintain a consistent position in the traffic flow.
• Maximum car: Driver maintains the highest possible speed under the legal limit.
• Average car: Driver maintains the average traffic speed.

Field Study Techniques – License Plate

• Observers are stationed at the entry and exit points of segments to record vehicle data.

Field Study Techniques - Others

• Observations from a vantage point.
• Test cars might be equipped with speed and distance measuring devices.

Example (Spot Speed)

• Illustrative, real-world data is provided to illustrate the application of the concepts introduced.

Example 10.1

• Spot speed data from an uncongested rural highway location is presented for analysis.

10.1(a)

• Creating frequency and cumulative frequency graphs from presented data.

10.1(b)

• Determining median, modal, and pace from graphs/data

10.1(c)

• Calculate mean and standard deviation using presented data

10.1(d)

• Calculate confidence bounds for estimating true mean speed (95% and 99.7%).

10.1(e)

• Given tolerance, estimate sample size required for 95% confidence.

10.1(f)

• Assess whether the data can be considered normally distributed using a Chi-Square test.

Control Delay Measurements for Signals

• Methods for measuring control delay at signalized intersections, as described in the Highway Capacity Manual (HCM), specifying the need for two observers.

Control Delay Measurement

• Methods used by the first observer in measuring control delay

Control Delay Measurement

• Methods used by the second observer in measuring control delay.

Detailed formulas and equations for delay analysis

Critical Parameters

• Volume, rate of flow, demand, capacity
• Units: Veh/hr or pc/hr, may also be per lane

Volume Characteristics

• Demand can vary by time of day, day of the week, month/season, and singular events like construction or accidents.
• Intelligent Transportation System (ITS) technologies may be applied for real-time demand management.
• Driver awareness plays a role in influencing demand.

Intersection Volume Studies

• Turning Movement Counts (TMCs) provide accurate observations of vehicle movements.
• Observers should be positioned appropriately.
• Volumes are counted when vehicles depart the intersection.
• Inexperienced observers: one major, two minor observers
• Short-break and alternating-period approaches help reduce observer need in studies with high traffic volumes or time periods.

Arrival Versus Departure Volumes

• Departure volume isn't suitable when demand exceeds capacity.
• Unstable queue buildup scenarios are dependent on signal status and intersection types.
• Arrival volumes can be estimated using departure data in dynamic queue situations.

A Multiday Study

• Data for control points and coverage locations across several days are utilized for comprehensive traffic volume analyses.

Mixed Approach: Three-day Study

• Three-day study captures long-term trends.

Problem 10.3

• Estimating queue time and average control delay given intersection data.

Problem 10-4

• Estimating the number of travel time runs to achieve the specified tolerance and confidence level under different standard deviations.

Problem 10.5

• Summarized travel time data for five runs at different checkpoints on a roadway, along with delays and stop counts.

Part A

• Tabulate and graphically display results for travel time, average speed, and running speed for each segment/section.

Frequency and Cumulative Frequency Curves

• Visual representations of the collected traffic data

Freeway Design Example

• Designing a freeway considering specific conditions, like PHF and terrain, and required LOS(C) and D (minimum).

Problem 14-3

• Methods and formulas are used to determine a composite grade value from a compound grade profile

Calculating the Equivalent Volume in PC/H

• Calculation of equivalent volume given that the traffic composition includes trucks and RVs and is in a rolling terrain.

Example: HV Factors

• Determining the composite grade for a specified series of composite grades.

Calculating Maximum Flow Rates (Freeways and Multilane)

• Calculating the maximum flow rate for various levels of service and free-flow speeds.

Methods for determining Speed and Capacity Adjustment Factors

• Methods to adjust FFS and capacity based on factors including adverse weather, workzones and driver mix

Basic Freeways

• Highway characteristics and how they influence capacity analysis procedures

Multilane Highways

• Highway characteristics and how they influence capacity analysis procedures

Determining Adjustments for Heavy Vehicles

• Explaining how heavy vehicle equivalents are determined, noting the application of formulas and factors.

Effect of Terrain on Heavy Vehicle Factors (HV)

• Impact of terrain on heavy vehicle equivalents and how to calculate appropriate factors for different terrain types.

Types of Analysis

• Methods used in freeway and multilane highway capacity analysis: operational analysis, service rate/volume analysis, and design analysis

Operational Analysis

• How operational analysis defines conditions and determines the expected level of service and operating parameters of a given road section

Service Flow Rate and Service Volume Analysis

• Deriving service flow rates through use of service flow rate formulas with the help of maximum service flow rates.

Design Analysis

• Formula to determine the required number of lanes (one direction) needed to provide specific traffic-service level and conditions

Specialized Counting Studies

• Origin-Destination (O-D) studies; methods to determine origins and destinations of trips (license plates, postcards, and surveys/interviews)

Cordon Counts

• Definition, characteristics, and the importance of cordoned areas

Important Points (Cordon Analysis)

• Important aspects considered when defining and using cordon boundaries.

Additional Examples

• Illustrative examples of different data types to highlight methods in calculating traffic data.

Maximum Flow Rates for Basic Freeways

• Maximum flow rates for basic freeways depending on the speed-flow curve and FFS

Maximum Flow Rates for Multilane Highways

• Maximum flow rates for multilane highways depending on the speed-flow curve and FFS.

Daily Adjustment Factor Example and Calculation

• Illustrative example of the calculation of daily/monthly adjustment factors based on a permanent station count.

Additional Example (Freeway and Multilane)

• Calculations for additional freeway and multilane analysis scenarios.

Problem 14-4

• How to calculate an equivalent volume in pce/h given a freeway operating in rolling terrain with 12% trucks and 3% RVs, and a measured peak hour volume.

Adjusted Speed and Capacity

• Calculating adjusted speed and capacity using factors adjusted for weather, driver experience, and workzones.

CAF for Unadjusted FFS

• Estimating the capacity adjustment factor based on type of weather condition, visibility, and temperature.

SAF for Unadjusted FFS

• Estimating the speed adjustment factor based on type of weather condition, visibility, and temperature.

CAF for Incidents with Following Effects

• Establishing CAF values depending on incidences and number of lanes.

Driver Familiarity Classification

• Driver familiarity classification adjustment factors.

Workzone Effect

• Workzone effect adjustments calculated

Workzone Capacity, FFS, and Adjustment Factor

• Adjustment formulas are given for calculating work zone capacity and FFS values.

Examples (Highway Design)

• Illustrative examples of the use of the aforementioned formulas for the analysis of highway design situations.

This quiz focuses on the principles and practices of spot speed studies in traffic engineering. It covers the primary objectives, types of data collected, and factors considered during these studies. Test your understanding of this important area in traffic analysis.