Intersections 101
Shawn Leight
Intersections 101(in an hour!)
Intersection Geometric Design
Design vehicleTurning templatesSight distanceChannelizationTurn lane warrants
Operational AnalysisFHWA Traffic Analysis ToolboxTravel demand modelsAnalytical/deterministic toolsTraffic signal optimization toolsSimulation Models
Introduction Reference libraryHuman factors
Intersection PlanningSketch-planning toolsAccess management Non-traditional designs
Other TopicsSafety analysisTraffic calming Traffic Signals (2nd Hour)Ped Accommodations (3rd Hour)
Reference LibraryControl GeneralEvaluationDesign
The Human Element
Human FactorsPrimary Driving Tasks
NAVIGATION
CONTROL
GUIDANCE
Guidance/Control Process
Design and Location of Traffic Control Devices
1. Timing: – Provide information at a steady rate – Spread out lower priority information
2. Primacy:– Place sufficient target value on more urgent devices– Example: Flashing light on warning sign
3. Expectancy:– Drivers respond more correctly to expected situations – Give advance notice of an unexpected situation
4. Redundancy:– Provide critical information in more than one way – Pavement markings supplement signs
Timing:What’s wrong with this picture?
Source: PTOE Refresher Course
Primacy:What’s wrong with this picture?
Camden County Courthouse
Expectancy:What’s wrong with this picture?
Tipperary Hill in western Syracuse
Redundancy:What’s wrong with this picture?
Forest Park
Better?
Schuetz at Page
Turning Templates
Turning Path Issues
Results of Poor Turning Paths
Maryland Heights, Missouri
Sight Distance
Route M in Jefferson County, Missouri
Intersection Sight Distance
Intersection Sight Distance• Case A; No Control
– Assume both vehicles can stop or adjust speed before intersection– 2 second perception/reaction time and 1 second maneuver time
• Case B; Stop Control– Assume stopped vehicle can cross intersection or enter traffic stream safely from
stop.– 3 Cases: Left-turn, Right-turn, Cross– Assume non-yielding vehicle travels at prevailing speed
• Case C; Yield Control– Assume yielding vehicles can stop or adjust speed before intersection AND
stopped vehicle can cross intersection or enter traffic stream safely from stop.– 3 Cases: Left-turn, Right-turn, Cross– Assume non-yielding vehicle travels at prevailing speed
• Case D; Signals– Depending on protected/non-protected movements
• Case E; All way stop– Drivers need to be able to see each other
• Case F; Left-turn from Major Road– Similar to yield case
Case A; No Control
Case B; Stop Control
Case B; Stop Control
250 feet
Advanced Flashers
Forest Park Parkway, St. Louis
150 feet
Upper Bottom Road, St. Charles
9 Principles of Channelization• Discourage or prohibit undesirable or wrong-way movements • Clearly define desirable vehicle paths • Encourage desirable and safe vehicle speeds • Separate points of conflict• Traffic streams should cross at near-right angles and merge
at flat angles.• Facilitate the movement of high priority traffic flows.• Facilitate the traffic control scheme.• Accommodate decelerating, slow, or stopped vehicles outside
higher-speed through traffic lanes.• Provide safe refuge ( e.g., pedestrians and handicapped)
Near Eglin AFB Florida
Discourage or prohibit undesirable or wrong-way movements
Near Eglin AFB Florida
Clearly define desirable vehicle paths
Near Eglin AFB Florida
Encourage desirable and safe vehicle speeds
Near Eglin AFB Florida
Separate points of conflict
Near Eglin AFB Florida
Cross at near-right angles and merge at flat angles
Near Eglin AFB Florida
Facilitate the movement of high priority traffic flows
Near Eglin AFB Florida
Facilitate the traffic control scheme
Near Eglin AFB Florida
Accommodate decelerating, slow, or stopped vehicles outside higher-speed through traffic lanes
Near Eglin AFB Florida
Provide safe refuge ( e.g., pedestrians and handicapped)
Kingshighway at BJC/Forest Park
Access Management
Source: http://www.accessmanagement.govIndiana
What is Access Management?• “Access Management is the process that provides access
to land development while simultaneously preserving the flow of traffic on the surrounding road system in terms of safety, capacity, and speed.”
(Federal Highway Administration)
• Access Management seeks to limit and consolidate accessalong major roadways, while promoting a supporting street system and unified access and circulation systems for development. The result is a roadway that functions safelyand efficiently for its useful life, and a more attractive corridor.
(Transportation Research Board)
Why Implement Access Management?
• Up to 50% crash reduction
• 23-45% capacity increase
• 40-60% delay reduction• Better business
environment• Improved quality of life• Save tax dollars
Source: Michigan DOT
Camdenton
Access Management Tools• Access rights• Intersection spacing• Access management
standards/guidelines• Driveway controls • Turning restrictions• Frontage/backage roads• Shared/cross access• Comprehensive planning• Zoning ordinances• Site plan development• Permitting
Planning Example
• Maryland Heights Howard Bend Levee District
• 25M ft2 planned development
• Regional park• Comprehensive plan
– 6-lane expressway– Parallel collector roadway – Extensive Coordination:
• Developers, Land Owners, Levee District, Existing Businesses, St. Louis County and MoDOT
Backage Roads Near US12/18 in Madison, WI
Right in-Right out Intersection
Turn Lane Warrants
Safety Analysis
CRASH RATE
SEGMENT CRASH RATE
SPOT CRASH RATE
LVTARSEC ××××
=365
108
VTARSPOT ×××
=365
106
Crash Diagram
Intersection Configurations
New Jersey“Near Side” Jug Handle
New Jersey“Far Side” Jug Handle
New Jersey“Far Side” Jug Handle
Route 141 at Vance?
Median Treatments
Three/Quarter Access Intersection
Three/Quarter Access Intersection
Median U-turns
Median U-turns
Median U-turns• Advantages
– Reduced Crash Potential – Increased Cross-street Capacity– Easily Retrofitted – Medium/Low Cost
• Disadvantages– Indirect Movements
• Unfamiliar to Drivers• Public Education
– Mainline Weaving
New Designs
I-270 and Olive Boulevard, St. Louis
Continuous Flow Intersection
St. Louis CFIRoute 30 at Summit
St. Louis CFIRoute 30 at Summit
Photo courtesy of MoDOT
Diverging Diamond Intersection
Proposed at I-270 and Dorsett Road
Traffic calming
Forest Park
Traffic Calming Tools Available
• Vertical Deflections, Horizontal Shifts,and Roadway Narrowings are intended to reduce speedreduce speed and enhance the street enhance the street environmentenvironment for non-motorists
• Closures (diagonal diverters, half closures, full closures, and median barriers) are intended to reduce cutreduce cut--through trafficthrough traffic by obstructing obstructing traffic movementstraffic movements.
Potential Impacts:• can impact parking and driveway access • reduces pedestrian crossing width and
increases visibility of pedestrian • speeds reduced on average by 4% for two-
lane chokers and 14% for one lane chokers • bicyclists prefer not to have the travelway
narrowed into path of motor vehiclesEmergency Response Issues: • preferred traffic calming device by many
emergency response agencies
Chokers and Neck Downs
Raised Intersections
Potential Impacts: • reduction in through movement speeds at
intersection • reduction in midblock speeds typically less
than 10% • access not effected• entire intersection more pedestrian-friendly • no data available on safety impactsEmergency Response Issues: • slows emergency vehicles to approximately 15
mph
Full and Partial Closures
Potential Impacts: • concern over effects on emergency
response, street network connectivity, and parallel local streets that carry diverted traffic
• may divert significant traffic volumes • no significant effect on vehicle speeds
beyond the full street closureEmergency Response Issues: • can be designed to allow emergency
vehicle access
Turn Restrictions and One Way Systems
Diverters
Intersection Evaluation
FHWA Traffic Analysis Toolbox
• Traffic Analysis Tools Website– http://ops.fhwa.dot.gov/trafficanalysistools/toolbox.htm– Traffic Analysis Tools Primer– Decision Support Methodology for Selecting
Traffic Analysis Tools– Guidelines for Applying Traffic Microsimulation
Modeling Software
Traffic Analysis ToolsFHWA (Traffic Analysis Tools Primer)
• Sketch-planning tools• Travel demand models• Analytical/deterministic tools• Traffic signal optimization tools• Simulation Models
Sketch-Planning Tools“Back of the Napkin” Techniques
• Order of-magnitude estimates – travel demand and traffic operations
• Not considered to be a substitute for the detailed engineering analysis
• Cheap/easy – uses highly aggregated data• Limited in scope, analytical robustness, and
presentation capabilities
Travel Demand Models
• Some analytical capabilities– prediction of travel demand – consideration of destination choice, mode choice, time-of-day travel
choice, and route choice– representation of traffic flow in the highway network
• Developed to determine the benefits and impact of major highway improvements - not designed for detailed operational evaluation
• Limited capabilities to estimate changes in operational characteristics (speed, delay, & queuing)
Analytical/Deterministic ToolsHCS+, aaSIDRA, SYNCHRO
• HCM Implementation• Quickly predicts capacity, density, speed, delay, and
queuing • Validated with field data, laboratory test beds, or small-
scale experiments• Good for analyzing the performance of isolated or small-
scale transportation facilities• Limited in their ability to analyze network or system effects
Signal Optimization ToolsSYNCHRO, TRANSYT 7-F, & Others
• Designed to develop optimal signal-phasing and timing plans • May include capacity calculations; cycle length; splits
optimization, including left turns; and coordination/offset plans.
• Some optimization tools can also be used for optimizing ramp metering rates for freeway ramp control.
Simulation ModelsVISSIM, CORSIM, and SimTraffic
• Movement of individual vehicles – car-following and lane-changing theories – statistical distributions (a stochastic process)
• Tracked through the network over small time intervals– (e.g., 1 second or a fraction of a second).
• Upon entry, each vehicle is assigned a destination, a vehicle type, and a driver type.
• Computer time and storage requirements for microscopic models are large, – limits the network size and the number of simulation runs that
can be completed
Selection Criteria• Geographic scope
– isolated intersection, single roadway, corridor, or network.• Facility types
– freeways, high-occupancy vehicle (HOV) lanes, ramps, arterials, toll plazas, etc.
• Travel modes– single-occupancy vehicle (SOV), HOV, bus, train, truck, bicycle, and
pedestrian traffic.• Traffic management strategies and applications
– ramp metering, signal coordination, incident management, etc.• Driver Behavior
– Temporal, spatial, modal diversion/choice and induced/foregone demand
• Output – safety measures, efficiency, mobility, productivity, & environmental
measures• Cost-Effectiveness
– capital, effort, ease of use, hardware, data
When to use Microsimulation
Simulation Modeling Strengths (Chapter 31 - HCM 2000)
• Trial and error experimentation• Study system effects
– Interactive queuing processes– Multiple time periods– Vary demand over time and space
• Provides insight into key factors • Other approaches may not be appropriate• Stochastic results
Over-saturated mainlineOver-saturated mainline
SPILLBACK EFFECTS
Source: Grant ZammitFHWA Resource Center
SPILLOVER EFFECTS
Excessive left-turn queue blocks through lane
Excessive left-turn queue blocks through laneSource: Grant ZammitFHWA Resource Center
SPILLOVER EFFECTS
Excessive throughExcessive through--lane queue lane queue blocks entrance to turn laneblocks entrance to turn lane
Source: Grant ZammitFHWA Resource Center
FREEWAY BOTTLENECKS
Ramp queue blocks freeway laneRamp queue blocks freeway lane
Source: Grant ZammitFHWA Resource Center
Unique Driver Behavior
Closely Spaced Intersections• Simplistic HCM Methodologies• Limited HCM Output
– Spill Back Queues and Starvation
Simulation Modeling Shortcomings (Chapter 31 - HCM 2000)
• Extensive input requirements• Calibration/validation efforts and data• Analyst multidisciplinary training requirements
– Traffic flow theory– Computer programming– Probability and statistics
• Easier ways to solve problem
How does Simulation Compare to HCM Analysis?
• Paper presented at TRB in 2003– Comparison of Simulation Models and the HCM
• Compared HCM 2000 and CORSIM, INTEGRATION, MITSIMLab, Paramics, VISSIM, and WATSIM.
• “Modeling” was done by Software Developers
Study Goals• Questions Asked:
– How do the results of compare?– What is the range of results?– How do changes in demand affect results?– What calibrate techniques are used?
• Questions NOT Asked:– Which simulation model is “best”?– Which is better – simulation or HCM?– Which model is easiest to calibrate?– How should simulation be used “with” the HCM?
Study Segment
Model Comparison
Using the Toolbox
Break