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.