Geometric Features Related to Substantive Safety at Intersections

1
Geometric Features Related to Substantive Safety
at Intersections

• Configuration - Number of Legs
• Access near Intersections
• Median Openings for U-Turns and Left-Turns
• Left and Right Turn Lanes
• Shoulder Widening
• Intersection Sight Distance
• Horizontal and Vertical Alignment
• Angle of Intersection (Skew)
• Splitter Islands (Channelization)

2
Potential Conflict Points
4-leg versus 3-leg Intersections -- Safety vs.
Mobility/Efficiency
Cross intersection has 32 points, Offset T has
22 points
3
Number of Intersection Legs

• Collision rates at Offset pair of 2-T low
  through side road volume intersections are 43
  of the crash rate of one 4-approach intersection
• CRF 57
• Safety of 3-leg intersections increases as minor
  road traffic increases

Tried
NCHRP 500, Strategy 17.2 B4 Convert 4-legged
to two Offset T Intersections
4
Pair of Off-Set T Intersections
NCHRP 500, Strategy 17.1 B14
Wrong Orientation
Tried
Right Orientation
5
Number of Intersection Legs

• Collision rates for intersections with more than
  4 approaches are 2 to 8 times greater than for 4
  approach Intersections

NCHRP 500, Strategy 17.1 B14
6
Adjacent Driveway Closures/ Relocations for Rural
Intersections

• Access points within 250 feet upstream and
  downstream of an intersection are undesirable
• Consolidate multiple access points
• Relocate access to the adjacent side road if
  possible

Tried
NCHRP 500, Objective 17.1A Improve Management
of Access Near Unsignalized Intersections
7
Adjacent Driveway Closures/ Relocations for Rural
Intersections
Tried
NCHRP 500, Objective 17.1A Improve Management
of Access Near Unsignalized Intersections
8
Roadway Segment Crash Rate as a Function of ADT
and Access Density
Tried
NCHRP 500, Objective 17.1A Improve Management
of Access Near Unsignalized Intersections
9
Roadway Segment Crash Rate as a Function of ADT
and Access Density
Tried
FHWA-RD-91-044 Nov 1992
NCHRP 500, Objective 17.1A Improve Management
of Access Near Unsignalized Intersections
10
Crash Rates vs. Access Density
11
Before and After Access Management Project
12
Left Turn Lanes in the Rural Highway Environment

• Left turn lanes remove stopped traffic from
  through lanes
• mitigate rear-end conflict
• enable selection of safe gap

Proven

• Capacity is generally not the issue

NCHRP 500, Strategy 17.1 B1 Provide Left-Turn
Lanes
13
Crash Reduction Presence of Left Turn Lanes
Proven
NCHRP 500, Strategy 17.1 B1 Provide Left-Turn
Lanes
14
Crash Reduction Presence of Left Turn Lanes
Proven
NCHRP 500, Strategy 17.1 B1 Provide Left-Turn
Lanes
15
Offset Left-Turn Lane Geometry
NCHRP 500, Strategy 17.2 B1 Provide Positive
Offset for Left-Turn Lanes
16
Offset Left-Turn Lane Geometry
17
Offset Left-Turn Lane Geometry
18
Offset Left Turn Lanes
Florida DOT very wide offsets
North Dakota DOT since 1999 wherever possible
19
Offset Left Turn Lanes
Phoenix - applies to undivided roads
20
Offset right turn lane Iowa adds island
Iowa
Curb encourages good driver sight line
21
Crash Reduction Presence of Right Turn Lanes
Proven
NCHRP 500, Strategy 17.1 B6 Provide Right
Turn Lanes
22
Offset right turn lane
Kentucky

• KY District 12
• anytime a new school goes in
• minimum 8 offset

23
Shoulder Widening at Rural Intersections

• Crash reduction per foot of shoulder widening at
  rural intersections

Tried
CRF 2.8/ft
NCHRP 500, Strategy 17.1 B10 Provide
Full-Width Paved Shoulders in Intersection Areas
24
Is this Intersection safer?
Discussion
BEFORE

• Where is the Sight Distance Limitation?
• What can you do about the Limitation?

25
Is this Intersection safer?
Discussion
AFTER

• Where is the Sight Distance Limitation?
• What can you do about the Limitation?

26
Criteria for the Leg of the Departure Sight
Triangle along the Major Road for Left and Right
Turns from STOP- controlled Approaches (Cases B1
and B2)
G -- Travel time gap (sec) at Design
Vehicle Design speed of major road
Passenger car 7.5 sec Single-unit truck
9.5 Combination truck 11.5
Exhibit 9-54, page 664 AASHTO Green Book
Adjustments for width of crossed facility and
grade on departure road
27
Intersection Sight Distance Suggested
Countermeasures

• Cut back vegetation to ISD values
• Remove sight obstructions of wall, fences, signs
• Establish 50 50 Clear Corner Sight Distance
  Policy/Ordinance
• Secure agreements with private property owners
• Restrict parking
• Move Stop Bars forward to just behind edge of
  outside lane
• Use Advisory Speed plaques on Approaches
• Reduce posted Speed Limit on Approaches
• Establish All-Way Stop Control per MUTCD

28
Poor Intersection Sight Distance Contributes to
Safety Problems
Tried

• 20 difference in Crash rate for Adequate
  Intersection Sight Distance

NCHRP 500, Strategy 17.1 C1 Clear Sight
Triangles at Stop and Yield Controlled
Intersections
29
Adequate Intersection Sight Distance Reduces
Crashes
Tried
AFTER
NCHRP 500, Strategy 17.1 C1 Clear Sight
Triangles at Stop and Yield Controlled
Intersections
30
Effect of Angle or Skew

• Some studies (McCoy, for example) show adverse
  effect of skew
• Skews increase exposure time to crashes increase
  difficulty of driver view at stopped approach

_at_ 90 degrees
Skew Angle
31
Crash Modification Factors -- Intersection Skew
Angle
Proven
For 3- legged Intersections
AMF exp (0.0040 SKEW)
For 4- legged Intersections
AMF exp (0.0054 SKEW)
SKEW Intersection Angle (degrees) as difference
between 90 degrees and actual intersection angle
NCHRP 500, Strategy 17.1 B16 Realign
Intersection Approaches
32
Crash Modification Factors -- Intersection Skew
Angle
Proven

• Intersection Skew from 90 degree sideroad for
  4-leg Approaches
• 0 15 30 45
• 1.00 1.08 1.18 1.27
• Max skew of 15 degrees - Older Driver Handbook
• Max skew of 30 degrees 2001 Green Book

NCHRP 500, Strategy 17.1 B16 Realign
Intersection Approaches
33
Solutions to Skewed Intersections
Proven
New Alignment
Old Alignment
NCHRP 500, Strategy 17.1 B16 Realign
Intersection Approaches
34
Solutions to Skewed Intersections

• Locate Intersection Away from Curve

CRF 25 Total Crashes
Proven
NCHRP 500, Strategy 17.1 B16 Realign
Intersection Approaches
35
Splitter Islands

• Install Splitter Islands on the Minor Road
  Approach to an Intersection

Tried

• Call Attention to the presence of the
  Intersection

NCHRP 500, Strategy 17.1 E3 Install Splitter
Islands on Minor Road Approaches
5 Year after (1999-2003) 4, all PDO, all
daytime
36
Painted Channelization
Painted channelization separating left turn lane
from thru lane

• Ensure visibility of intersection

Tried
NCHRP 500, Strategy 3.1 B8 Improve Roadway
Delineation
CRF 50 rural CRF 30 Suburban CRF 15 Urban
37
Raised Channelization
Raised channelization separating left turn lane
from thru lane

• Ensure visibility of intersection

Tried
NCHRP 500, Strategy 3.1 B9 Replace Painted
Channelization with Raised Channelization
CRF 60 rural CRF 65 Suburban CRF 70 Urban