Spot Speed Studies Overview

Spot Speed Studies

• Conducted for a point or a short segment (less than 1,000 feet).
• Used to determine driver speed choices under free flow conditions.
• Not performed 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

• Evaluating the effectiveness of existing or new speed limits and enforcement.
• Determining appropriate sight distances and relationships between speed and highway alignment features, grade steepness and length.
• Determining signal timing for "yellow" and "all red" intervals; proper placement of signs and signal progressions.
• Investigating high-accident locations.

Speed Definitions of Interest

• Average or time mean speed: The average speed of all vehicles passing a location during a study period.
• Standard deviation: The average difference between individual observed speeds and the average speed.
• 85th percentile speed: The speed below which 85% of observed vehicles travel.
• Median speed: The speed that divides the distribution of spot speeds in half.
• Pace: A 10 mph increment encompassing the highest proportion of observed speeds.

Speed Data Collection

• Collected using permanent detector locations (e.g., loop detectors).
• Collected using handheld or vehicle-mounted radar guns or detectors.
• Individual observed speeds are arranged into frequency distributions within defined speed groups.

Travel Time Studies

• Also known as Journey Time Surveys (JTS).
• Involves measuring travel times across significant lengths of highway segments or routes.
• Coordinated with delay studies.
• Used in: Identifying problem locations, Measuring arterial LOS, Providing input for traffic assignment models, Providing travel-time data for economic evaluation, and Developing time contour maps.

Field Study Techniques – Test Car

• Test cars are used for timed measurements of travel time, stops, delays and causes of delays.
• Automated equipment may be used
• Floating car: maintain position in the traffic stream
• Maximum car: drive as fast as possible
• Average car: drive at average speed of traffic

Field Study Techniques – License Plate

• Observers positioned at entry and exit points of segments.
• Sample size is an important factor.

Field Study Techniques – Others

• Observations from a vantage point.
• Equipment for speed and distance measurement.
• Video recording of the section.

Intersection Delay Studies

• Measuring effectiveness of signalized, stop-controlled, and roundabout intersections.
• Control delay: Time spent in queue plus deceleration/acceleration time to/from ambient speed.

Control Delay Measurement for Signals

• HCM (2000) specifications.
• Requires two observers.
• Intended for under-saturated conditions (queue ≤20-25 vehicles); observation begins during the red phase with no overflow from the previous cycle.
• Observers must identify the Free Flow Speed (FFS).

Control Delay Measurement

• Observer 1: Tracks ending queues at 10-20 second intervals (integral divisor of cycle length).

• Identifies vehicles still in the queue: vehicles that are still within the intersection.

• Monitors through movements when rear wheels cross stop lines and turning vehicles when wheels pass the opposing pedestrian space.

• Counts until end of survey period, all vehicles in queue during the survey period have exited the intersection.

• Observer 2: Separate counts of vehicles arriving and stopping, throughout the survey period.

Volume Studies

• Critical parameters: Volume, Rate of flow, Demand, Capacity.
• Volume measured in veh/hr or pc/hr (per lane).

Volume Characteristics

• Demand varies by time of day, day of the week, month/season.
• Response to singular events (e.g., construction, accidents) and severe weather.
• Modern ITS (intelligent transportation systems) technologies provide real-time demand management.

Intersection Volume Studies

• Turning Movement Counts (TMCs).
• Manual counting, accurate observation of different movements.
• Appropriate observer positioning.
• Counting volumes as vehicles depart the intersection.
• Inexperienced observers need one major or two minor.
• Lane-wise counting for heavily used multilane approaches.
• Short-break and alternating-period approaches reduce observer requirements.

Special Considerations for Signalized Intersections

• One observer can monitor more than one movement.
• Count periods and actual counting times must be equal multiples of the cycle length.
• Actuated signals require counting periods of at least 5 times the maximum cycle length.

Limited Network Volume Study

• Affected by major traffic generators, making it difficult to perform personal full counts across an entire network.
• Expensive to acquire sufficient portable equipment and train personnel.
• Employ sampling techniques.

Sampling Procedures

• Sampling assumption: entire networks, or identifiable sub portions of networks, have similar patterns of demand in time.
• Sampling counts: includes both control and coverage counts.

Guidelines for Volume Counts

• One control count location for every 10 to 20 coverage count locations.
• Different control-count locations based on facility type and significant differences in land-use characteristics.
• Counts should be taken at mid-blocks, and each link at least once during the study period.
• Intersection movements may be inferred from surrounding link movements.

Statewide Counting Programs

• Focus on determining AADT (average annual daily traffic), shifts within the ADT pattern, and vehicle-miles traveled.
• State road system divided into functional classifications to establish patterns of control and coverage count locations.
• Similar to limited network studies, but with the entire state highway system, and continuous time frames for the study.
• Guidance to conduct a coverage count every year on every 2-mile segment of the state highway system.
• One control-count location for every 20-50 coverage-count locations.

Bahrain Counting Points – 2015

• Mapping of temporary and permanent count locations.

Basic Guidelines for Count Locations

• Permanent locations use fixed detection equipment.
• Major and minor control counts use portable counters and tubes.

Grouping Data from Control Locations

• Possible for a broad region to have similar (or identical) daily and/or monthly adjustment factors.
• Spatially contiguous control stations on the same classification of highway may be grouped together and data pooled.
• Use the average adjustment factors of the group for a larger area.

Specialized Counting Studies

• Origin-destination counts.
• Cordon counts.
• Screen-line counts.

Cordon Counts

• Imaginary boundary around a study area of interest; often used to define the central business district (CBD) or other major activity centers.
• Accumulation of vehicles within the area is of great importance.
• Considerations: size (large enough but not too large), geographic uniformity of land use.

Important Points for Cordons

• Large enough to define the whole area of interest.
• Small enough for accumulation estimates to be useful for parking and other traffic planning purposes.
• Cordoned areas should have relatively uniform land uses.
• Establishing the cordon to intersect all streets and highways at mid-blocks.
• Using natural or manufactured barriers to minimize crossing points.

Screen-Line Counts

• Convenient barriers that divide up a study area with limited crossings.
• Natural barriers can be used for screen lines.
• Similar to Origin-Destination (OD) counts, but with only two zones.

King Fahd Causeway Example

• Data example for the screen line between Saudi Arabia and Bahrain showing daily traffic counts.

Capacity and Level of Service for Uninterrupted Facilities

• Defining the HCM (Highway Capacity Manual).
• HCM is the US standard for capacity and level of service analyses.
• Published by the Transportation Research Board (TRB) of the National Academy of Engineering.
• Published in: 1950, 1965, 1985, 2000, 2010, 2016, and 2022.

Highway Capacity Manual

• The principal implementing software for the capacity and level of service analyses since 1994.
• Maintained by the McTrans Center at the University of Florida in Gainesville.
• HCM methodology adopted by other software development companies (e.g., VISSIM, Synchro, PARAMICS, SIDRA).

Capacity

• Maximum hourly rate of flow of persons or vehicles reasonably expected to traverse a point or a uniform section.

Level of Service (LOS)

• Quality measure of operational conditions in terms of traffic stream speed, travel time, freedom of maneuver, traffic interruptions, comfort, and convenience.
• Defined using a letter-grade ("A" to "F") scale for indicating the quality of operational conditions.

LOS Levels

• LOS E: capacity operations.
• LOS D: maximum sustainable flow.
• LOS C: practical capacity for urban facilities.
• LOS B: practical capacity for rural facilities.
• LOS A: high operating quality.

Basic Freeways

• Pure uninterrupted flow facilities.
• All entries and exits are made via ramps, without interruptions.
• No at-grade intersections, driveways, or parking.
• Classified by total number of lanes.

Multilane Highways

• Considered uninterrupted flow facilities (when signal spacing is more than 2 miles).
• Classified by number of lanes and median type (divided or undivided).

Capacity Analysis for Freeways and Multilane Highways

• Based on Free Flow Speed (FFS) when flow is less than 1000 veh/hr/ln.
• Use calibrated speed-flow curves for various free-flow speeds.
• Base conditions include no heavy vehicles and a driver population dominated by those frequent and familiar with the facility.

LOS A

• Free flow; not influenced by other vehicles.
• Speed is not affected by flow; lane changing and other maneuvers are easily accomplished.
• Short-duration blockages.
• Average spacing 480 ft (24 car lengths)

LOS B

• Still free flow; drivers can respond to other vehicles.
• Maneuvering is relatively easy, but vigilance is required in response to other vehicles.
• Still sufficient gaps are to dampen minor disruptions.
• Average spacing 293 ft (15 car lengths).

LOS C

• Still free flow; maneuverability is restricted due to other vehicles.
• Drivers need to adjust their course to find gaps.
• Significant increase in required driver vigilance.
• Significant disruptions may cause total breakdown.
• Average spacing 203 ft (10 car lengths).

LOS D

• Average speeds begin to decline with increased flows.
• Density deteriorates more quickly in response to small increases.
• Maneuvering within the traffic stream is difficult.
• Significant lane disruptions have more negative impact.
• Average spacing 151 ft (7 car lengths).

LOS E

• Near capacity; few usable gaps.

• Perturbations cause shock waves and extensive queuing problems.

• Maneuvering is very difficult.

• Other vehicles must yield to accommodate lane changes.

• Average spacing 117 ft (6 car lengths).

Prevailing Condition

• Lane widths; lateral clearances.
• Types of medians (multilane highways).
• Frequency of ramps, access points (multilane highways).
• Presence of heavy vehicles.
• Driver populations familiar with type of facility.

Types of Analysis

• Operational analysis.
• Service flow rate and service volume analysis.
• Design analysis.

Operational Analysis

• Existing or future highway conditions are described to analyze the expected service level and operating parameters for the existing or proposed highway section.
• An expression that relates demand (flow rate per lane) with the peak-hour factor (PHF), number of lanes (N), and adjustment factor (fHV)

Service Flow Rate and Service Volume Analysis

• Service flow rate for a particular level of service.
• Maximum service flow rate for a specific level of service.
• Service volume per a given peak hour for a particular level of service.

Determining Adjustment Factor for Heavy Vehicles

• Heavy vehicle defined as vehicle with 4+ touching tires to the pavement.
• Passenger car equivalent is the number of passenger cars that is equivalent to a heavy vehicle.

This quiz focuses on the essential concepts of spot speed studies in traffic engineering. It covers the primary purpose of these studies, scenarios for their effectiveness, data gathered, and factors influencing results. Test your knowledge in this critical area of transportation analysis.