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Publication Number: FHWA-RD-02-045
Date: March 2003

IHSDM Intersection Diagnostic Review Model

6.0  IDRM TREATMENT RECOMMENDATIONS

When IDRM identifies a potential intersection design concern, it also identifies design changes and other measures that could eliminate or mitigate the concern.  These are referred to as "treatments."  Treatments that could mitigate each identified concern are presented to the user through the IDRM user interface.

The IDRM knowledge base relates concerns to the applicable treatments by means of the treatment recommendation matrix.  A row in this matrix is specified by a concern, a specific geometric condition or other design element related to the concern, and a treatment that addresses that condition.  For some treatments, the design element is designated as "general," indicating that the treatment is potentially applicable to the concern, regardless of the specific design element related to the concern.

Each treatment is categorized as either a "design improvement" (generally higher cost) or a "mitigation measure" (generally lower cost).

Table 26 and Table 27 present the IDRM treatment recommendation matrix for intersection concerns and intersection-leg concerns, respectively.

Table 26. List of Treatments (Design Improvements and Mitigation Measures) for Intersection Concerns

Concern Messages

Design Improvements

Mitigation Measures

4.20 Insufficient Left-Turn Bay Storage and Deceleration Length Between Closely Spaced Intersections

- back-to-back left-turn lanes

  • Relocate one or both minor-road legs
  • Close one minor-road leg
  • Provide an alternative path around the intersection for minor-road vehicles
  • Convert to back-to-back parallel left-turn lanes
  • Prohibit left turns
  • Restrict left turns
  • Signalize intersection (if unsignalized)
  • Convert to all-way stop
  • Restripe with shorter taper

4.21 Insufficient Taper Length for Left-Turn Lanes Between Closely Spaced Intersections

- back-to-back left-turn lanes

  • Convert back-to-back to parallel left-turn lanes
  • Relocate one or both minor-road legs
  • Close one minor-road leg
  • Provide an alternative path around the intersection for minor-road vehicles
  • Prohibit turns
  • Convert to all-way stop (if unsignalized)
  • Restripe with shorter taper

Table 26.  List of Treatments (Design Improvements and Mitigation Measures) for Intersection Concerns (continued)

Concern Messages

Design Improvements

Mitigation Measures

4.22 High Traffic Conflict Index

- multileg intersection

  • Relocate one or more legs
  • Make one or more legs one-way
  • Close one or more legs
  • Signalize (if unsignalized)
  • Prohibit turns
  • Restrict turns

4.24 Large Intersection Pavement Area

- multileg intersection

  • Add channelizing islands
  • Relocate one or more legs
  • Close one or more legs
  • Consider smaller design vehicle
  • Improve drainage
  • Consider roundabout
  • Supplemental dotted line
  • Add painted channelization

- skewed intersection

  • Realign one or more legs (consider offset T)
  • Add channelizing islands
  • Relocate one or more legs
  • Close one or more legs
  • Consider smaller design vehicle
  • Improve drainage
  • Supplemental dotted line
  • Add painted channelization

- large vehicle turn path

  • Realign approach
  • Rechannelize turn to accommodate design vehicle (if channelization present)
  • Increase throat width
  • Remove unnecessary channelization (if channelization present)
  • Consider smaller design vehicle
  • Move stop bar

 

Table 27.  List of Treatments (Design Improvements and Mitigation Measures) for Intersection-Leg Concerns

Concern Messages

Design Improvements

Mitigation Measures

4.1 - 4.7 Insufficient ISD to right (Case B1) for ___ leg

- general

  • Remove roadside obstacles within sight triangle
  • Close approach
  • Relocate approach
  • Make leg one-way away from intersection
  • Reduce upgrade on approach (if upgrade is present)
  • Remove roadside obstacles within sight triangle
  • Signalize intersection (if unsignalized)
  • Convert to all-way stop (if unsignalized)
  • Convert yield control to stop control (if yield- controlled)
  • Post advisory speed on major road
  • Review speed limit on major road
  • Install warning sign on major road
  • Install flashing beacons (if unsignalized)
  • Prohibit left turn
  • Provide intersection lighting

-  multileg intersection

  • Close one or more legs
  • Relocate one or more legs
 

-  skewed intersection

  • Realign one or more legs
  • Relocate one or more legs
  • Close one or more legs
 

- more than one minor-road approach on the same side of the major road

  • Close one or more legs
  • Relocate one or more legs
  • Make one or more legs one-way
 

- horizontal curve

  • Increase curve radius
  • Remove roadside obstacles on inside of curve
 

- crest vertical curve

  • Lengthen vertical curve
 

Table 27.  List of Treatments (Design Improvements and Mitigation Measures) for Intersection-Leg Concerns (continued)

Concern Messages

Design Improvements

Mitigation Measures

4.1 - 4.7 Insufficient ISD to left (Case B2) for ___ leg

- general

  • Remove roadside obstacles within sight triangle
  • Close approach
  • Relocate approach
  • Make leg one-way away from intersection
  • Reduce upgrade on approach (if upgrade is present)
  • Install channelized right-turn roadway
  • Provide right-turn acceleration lane
  • Remove roadside obstacles within sight triangle
  • Signalize intersection (if unsignalized)
  • Convert to all-way stop (if unsignalized)
  • Convert yield control to stop control (if yield-controlled)
  • Post advisory speed on major road
  • Review speed limit on major road
  • Install warning sign on major road
  • Install flashing beacons (if unsignalized)
  • Prohibit right turn
  • Provide intersection lighting
  • Restripe shoulder as right-turn acceleration lane

- multileg intersection

  • Close one or more legs
  • Relocate one or more legs
 

- skewed intersection

  • Realign one or more legs
  • Close one or more legs
  • Relocate one or more legs
 

- more than one minor-road approach on the same side of the major road

  • Close one or more legs
  • Relocate one or more legs
 

- horizontal curve

  • Increase curve radius
  • Remove roadside obstacles on inside of curve
 

- crest vertical curve

  • Lengthen vertical curve
 

Table 27.  List of Treatments (Design Improvements and Mitigation Measures) forIntersection-Leg Concerns (continued)

Concern Messages

Design Improvements

Mitigation Measures

4.1 - 4.7 Insufficient ISD to right (Case B3) for ___ leg

- general

  • Remove roadside obstacles within sight triangle
  • Close approach
  • Relocate approach
  • Make leg one-way away from intersection
  • Reduce upgrade on approach (if upgrade is present)
  • Remove roadside obstacles within sight triangle
  • Signalize intersection (if unsignalized)
  • Convert to all-way stop (if unsignalized)
  • Convert yield control to stop control (if yield-controlled)
  • Post advisory speed on major road
  • Review speed limit on major road
  • Install warning sign on major road
  • Install flashing beacons (if unsignalized)
  • Prohibit left turn
  • Channelize to prohibit left turns and through movements
  • Provide intersection lighting

- multileg intersection

  • Close one or more legs
  • Relocate one or more legs
 

- skewed intersection

  • Realign one or more legs
  • Relocate one or more legs
  • Close one or more legs
 

-  more than one minor-road approach on the same side of the major road

  • Close one or more legs
  • Relocate one or more legs
  • Make one or more legs one-way
 

- horizontal curve

  • Increase curve radius
  • Remove roadside obstacles on inside of curve
 

- crest vertical curve

  • Lengthen vertical curve
 

Table 27.  List of Treatments (Design Improvements and Mitigation Measures) for Intersection-Leg Concerns (continued)

Concern Messages

Design Improvements

Mitigation Measures

4.1 - 4.7 Insufficient ISD to left (Case B3) for ___ leg

-  general

  • Remove roadside obstacles within sight triangle
  • Close approach
  • Relocate approach
  • Make leg one-way away from intersection
  • Reduce upgrade on approach (if upgrade is present)
  • Install channelized right-turn roadway
  • Provide right-turn acceleration lane
  • Remove roadside obstacles within sight triangle
  • Signalize intersection (if unsignalized)
  • Convert to all-way stop (if unsignalized)
  • Convert yield control to stop control (if yield-controlled)
  • Post advisory speed on major road
  • Review speed limit on major road
  • Install warning sign on major road
  • Install flashing beacons (if unsignalized)
  • Prohibit right turn
  • Channelize to prohibit through movements
  • Provide intersection lighting
  • Restripe shoulder as right-turn acceleration lane

- multileg intersection

  • Close one or more legs
  • Relocate one or more legs
 

- skewed intersection

  • Realign one or more legs
  • Close one or more legs
  • Relocate one or more legs
 

- more than one minor-road approach on the same side of the major road

  • Close one or more legs
  • Relocate one or more legs
 

- horizontal curve

  • Increase curve radius
  • Remove roadside obstacles on inside of curve
 

- crest vertical curve

  • Lengthen vertical curve
 

Table 27.  List of Treatments (Design Improvements and Mitigation Measures) for Intersection-Leg Concerns (continued)

Concern Messages

Design Improvements

Mitigation Measures

4.1 - 4.7 Insufficient ISD for left turn from major road (Case F) for ___ leg

- general

  • Close departure roadway
  • Relocate departure roadway
  • Make departure roadway one-way toward intersection
  • Provide left-turn lane on major road
  • Provide left-turn lanes with positive offset
  • Prohibit left turns
  • Restrict left turns
  • Signalize and provide exclusive left-turn lane signal phase (if unsignalized)
  • Provide exclusive left-turn signal phase
  • Post advisory speed on major road
  • Review speed limit on major road
  • Install warning sign on major road
  • Install flashing beacons (if unsignalized)
  • Provide intersection lighting

- multileg intersection

  • Close one or more legs
  • Relocate one or more legs
 

- skewed intersection

  • Realign one or more legs
  • Relocate one or more legs
  • Close one or more legs
 

- more than one minor-road approach on the same side of the major road

  • Close one or more legs
  • Relocate one or more legs
  • Make one or more legs one-way
 

- horizontal curve

  • Increase curve radius
  • Remove roadside obstacles on inside of curve
 

- crest vertical curve

  • Lengthen vertical curve
 

Table 27.  List of Treatments (Design Improvements and Mitigation Measures) for Intersection-Leg Concerns (continued)

Concern Messages

Design Improvements

Mitigation Measures

4.8 - 4.9 Insufficient SSD for ___ leg

- horizontal curve

  • Increase curve radius
  • Remove roadside obstacles on inside of curve
  • Close intersection
  • Relocate intersection
  • Lengthen intersection approach channelization (if left-turn lane present)
  • Provide left-turn lane (if no left-turn lane present)
  • Post advisory speed on curve
  • Reduce speed limit
  • Install warning sign
  • Prohibit turns
  • Install advance warning beacon and sign

- crest vertical curve

  • Lengthen vertical curve
  • Close intersection
  • Relocate intersection
  • Lengthen intersection approach channelization (if left-turn lane present)
  • Provide left-turn lane (if no left-turn lane present)
  • Post advisory speed on curve
  • Reduce speed limit
  • Install warning sign
  • Prohibit turns
  • Install advance warning beacon and sign

-  multileg intersection

  • Close one or more legs
  • Relocate one or more legs
 

- more than one minor-road approach on the same side of the major road

  • Close one or more legs
  • Relocate one or more legs
  • Make one or more legs one-way
 

4.10 Insufficient Visibility of Traffic Signal

- general

  • Realign approach
  • Remove roadside sight obstructions
  • Move traffic control device
  • Install advance warning sign
  • Provide supplemental traffic control device
  • Install advance warning beacon and sign

4.11 Insufficient Visibility of Stop Sign

- general

  • Realign approach
  • Remove roadside sight obstructions
  • Move traffic control device
  • Install advance warning sign
  • Provide supplemental traffic control device
  • Install advance warning beacon and sign

Table 27.  List of Treatments (Design Improvements and Mitigation Measures) for

Intersection-Leg Concerns (continued)

Concern Messages

Design Improvements

Mitigation Measures

4.12 Insufficient Visibility of Yield Sign

- general

  • Realign approach
  • Remove roadside sight obstructions
  • Move traffic control device
  • Install advance warning sign
  • Provide supplemental traffic control device
  • Install advance warning beacon and sign

4.13 - 4.16 Insufficient DSD for ___ leg

- horizontal curve

  • Increase curve radius
  • Remove roadside obstacles on inside of curve
  • Close intersection
  • Relocate intersection
  • Lengthen intersection approach channelization (if left-turn lane present)
  • Provide left-turn lane (if no left-turn lane present)
 

- crest vertical curve

  • Lengthen vertical curve
  • Close intersection
  • Relocate intersection
  • Lengthen intersection approach channelization (if left-turn lane present)
  • Provide left-turn lane (if no left-turn lane present)
 

-  multileg intersection

  • Close one or more legs
  • Relocate one or more legs
 

- more than one minor-road approach on the same side of the major road

  • Close one or more legs
  • Relocate one or more legs
  • Make one or more legs one-way
 

Table 27.  List of Treatments (Design Improvements and Mitigation Measures) for Intersection-Leg Concerns (continued)

Concern Messages

Design Improvements

Mitigation Measures

Increased Crossing Distance

- skewed intersection

  • Realign one or more legs
  • Relocate one or more legs
  • Close one or more legs
  • Signalize intersection (if unsignalized)
  • Convert to all-way stop (if unsignalized)
  • Convert yield control to stop control (if yield-controlled)
  • Post advisory speed on major road
  • Reduce speed limit on major road
  • Install warning sign on major road
  • Install flashing beacons (if unsignalized)
  • Provide lighting
  • Increase signal clearance on all-red time (if signalized)

4.18 Warranted Left-Turn Lane Is Not Present

- general

  • Provide left-turn lane
  • Provide shoulder bypass lane (if three-leg intersection and unsignalized)
  • Provide warning signs
  • Prohibit left turns
  • Restrict left turns
  • Provide split phasing (if signalized)
  • Restripe or reallocate approach lane configuration

4.19 Warranted Right-Turn Lane Is Not Present

- general

  • Provide right-turn lane
  • Provide channelized right-turn roadway
  • Provide right-turn acceleration lane
  • Restripe or reallocate approach lane configuration
  • Install advance warning sign

4.23 Uneven, Discontinuous Minor-Road Profile Through Intersection

- general

  • Redesign minor-road profile through intersection
  • Provide warning signs
  • Spot resurfacing

Table 27.  List of Treatments (Design Improvements and Mitigation Measures) for Intersection-Leg Concerns (continued)

Concern Messages

Design Improvements

Mitigation Measures

4.25 Approach Alignment Differs Between Opposing Approaches

- general

  • Realign one or more legs
  • Provide horizontal curve to eliminate deflection angle
  • Add painted channelization to delineate vehicle path
  • Review speed limit on approach legs
  • Post advisory speed signs on approach legs
  • Install flashing beacons (if unsignalized)
  • Convert to all-way stop (if unsignalized)
  • Provide intersection lighting

4.26 Insufficient Queue Storage

- general

  • Increase storage-bay length
  • Prohibit left turns
  • Restrict left turns
  • Signalize intersection
  • Convert to all-way stop
  • Restripe turn bay with a shorter taper length

4.27 Loss of Control Potential Due to Frequent Braking

- horizontal curve

  • Relocate intersection
  • Increase curve radius
  • Provide left-turn lane
  • Provide right-turn lane
  • Increase superelevation
  • Improve drainage
  • Provide more skid-resistant pavement
  • Post advisory speed
  • Reduce speed limit
  • Install warning sign
  • Increase signal clearance on all-red time (if signalized)

 

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