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