p> « back to results for ""
Below is a cache of http://www.ncdot.org/doh/preconstruct/traffic/conference/reports/uid1.pdf. It's a snapshot of the page taken as our search conventional-/' >engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
Moving More Cars Through the Same Space Using Unconventional Intersection Designs
Moving More Cars Through the
Same Space Using
Unconventional Intersection
Designs
Joseph E. Hummer, Ph.D., P.E.
phone 919-515-7733
hummer@eos.ncsu.edu
For NCDOT Traffic Engineering
Conference, May 14, 2003 Objective
Study the menu of unconventional
intersection designs, and
Learn the important advantages and
disadvantages of each design so that you
can
Specify an appropriate unconventional
intersection design for a particular problem
location Problem
Growing demand
Close to 50/50 directional split
Conventional solutions exhausted
Too expensive to widen
Structures expensive and unpopular
ITS, transit, demand management, etc. not
helpful Potential Solution:
Unconventional Designs
12 designs on current intersection menu
All 12 designs published
Most of the 12 designs in use in U.S.
This presentation discusses 8 of 12 with
most potential in NC Major Principles
Reduce delay to through vehicles
Reduce number of conflict points at
intersections
Separate remaining conflict points
Reduce signal phases
Accomplished mostly by rerouting left turns Driver Confusion?
Potential is there; however...
Most in place somewhere for years
Precedent in other new designs
Roundabout, single point diamond, etc.
Traffic control devices helpful
Design whole corridor Median U-Turn
Arterial
Arterial or collector Median U-Turn Advantages
Increased capacity
Reduced travel time
Enhanced progression
Fewer threats to pedestrians
Fewer conflict points
Lower collision rates Median U-Turn Capacity
Critical V/C, 30,000 ADT
Minor ADT
% turns
Med. U-turn Conventional
15,000
20
0.74
0.86
40
0.88
0.90
25,000
20
0.90
1.04
40
1.11
1.14 Typical Corridor Travel Time
MOE
TWLTL
Median U-
Turn
Travel time,
veh-hours
403
280
Stops per
vehicle
2.08
2.19 Median U-Turn Collision Rates
(per 100 mil. veh-miles)
Signalized? TWLTL
Conventional
Median
U-Turn
Yes
1220
750
600
No
460
180
220 Median U-Turn Disadvantages
Left turn delay
Left turn travel distance
Left turn stops
Wider right-of-way
Higher minimum green time for pedestrians
Indirect left turns into businesses
Wide median means less business visibility Median U-Turn Variations
Three-legged intersection--very efficient
Narrow median with turning bulbs
U-turns on minor conventional-/' target='blank' class='doin' >street
STOP sign for u-turns
Interchange Bowtie
Arterial
Collector Bowtie Advantages
Narrow major conventional-/' target='blank' class='doin' >street right-of-way
Short, simple pedestrian crossing
Enhanced major conventional-/' target='blank' class='doin' >street progression
Aesthetics
Developments can tie into roundabouts Bowtie Disadvantages
Low minor conventional-/' target='blank' class='doin' >street capacity
Left turn delay
Left turn travel distance
Left turn stops
Difficult arterial u-turn Bowtie Variation
Raindrop Interchange
Roundabouts instead of signals at ramp
terminals
Many in use around U.S. now
Safe
Efficient
Narrow bridge Superconventional-/' target='blank' class='doin' >street
Arterial
Collector
Pedestrians Superconventional-/' target='blank' class='doin' >street Advantages
Perfect two-way progression with any
signal spacing!
More efficient with light minor conventional-/' target='blank' class='doin' >street
volumes
Should be safer
All pedestrian crossing controlled
Works well on an arterial lined with
development Superconventional-/' target='blank' class='doin' >street Travel Time
MOE
TWLTL
Median U-
Turn
Superconventional-/' target='blank' class='doin' >street
Travel time,
veh-hours
403
280
314
Stops per
vehicle
2.08
2.19
2.59 Superconventional-/' target='blank' class='doin' >street Disadvantages
Less efficient with heavy minor conventional-/' target='blank' class='doin' >street
volumes
Wider right-of-way
Two stage pedestrian crossing
Indirect left turns into businesses
Wide median means less business visibility Superconventional-/' target='blank' class='doin' >street Variations
Three-legged intersection
One direct left turn
No direct left turns
One conventional intersection in corridor Continuous Flow Intersection
Arterial
Arterial or collector Continuous Flow Intersection
Advantages
Reduced travel time with high volumes
Keeps traffic moving
Enhanced progression
Narrower major conventional-/' target='blank' class='doin' >street ROW
Fewer conflict points Continuous Flow Intersection
Disadvantages
No u-turns at intersection
Pedestrians must cross ramps
Access difficult for parcels next to ramps Continuous Flow Intersection
Variations
Three-legged intersection
Ramps on one to four legs
Continuous Flow Interchange Single Quadrant
Arterial
Arterial or collector Single Quadrant Advantages
Typically vies with median u-turn as most
efficient unconventional design
Major and minor conventional-/' target='blank' class='doin' >streets can have narrow
rights-of-way
Connector road provides development
opportunity
Some pedestrians have shorter, simpler
crossing Single Quadrant Disadvantages
Some left turns have more travel time,
distance, stops
ROW for connector road
No u-turns at main intersection
No driveways opposite ends of connector
road
Some pedestrians must cross connector road
too Single Quadrant Variations
Which quadrant?
Two quadrants
Allow a direct left turn or two
Single quadrant interchange Continuous Green T
Arterial
Arterial or collector Continuous Green T Advantages
Lower travel times
Narrow ROW Continuous Green T
Disadvantages
Median design difficult
Right-in-right-out driveways only on top of
the T Continuous Green T Variation
from Florida
No median
On top of T, left lane signalized while right
lane has 100% green
Left lane signal shows red to allow left
turns from stem of T
Lane changing and enforcement issues? Echelon Interchange
Arterial
Arterial or collector Echelon Interchange Advantages
Much higher capacity than at-grade
intersections
Much lower travel time than at-grade
intersections
Enhanced progression for both conventional-/' target='blank' class='doin' >streets
Meters traffic to help downstream signals Typical critical volume/capacity ratios
0.99
1.19
80,000
0.86
1.03
70,000
0.75
0.89
60,000
Echelon
interchange
Median
u-turn
Intersection
volume,
veh/day Typical simulated peak hour
total travel times, hours
105
170
80,000
75
140
70,000
60
75
60,000
Echelon
interchange
Median
u-turn
Intersection
volume,
veh/day Echelon Interchange
Disadvantages
High structure cost
Access impaired to 3 quadrants
No u-turns at or near interchange
Pedestrians must climb grades or cross
conventional-/' target='blank' class='doin' >streets unprotected by signals Center Turn Overpass
Arterial
Arterial or collector Center Turn Overpass
Advantages
Much higher capacity than at-grade intersections
Much lower travel time than at-grade intersections
Enhanced progression for both conventional-/' target='blank' class='doin' >streets
Meters traffic to help downstream signals
Direct pedestrian crossing
Access to roadside businesses similar to
conventional intersection with medians Typical critical volume/capacity ratios
1.06
0.99
1.19
80,000
0.93
0.86
1.03
70,000
0.80
0.75
0.89
60,000
Center turn
overpass
Echelon
interchange
Median
u-turn
Intersection
volume,
veh/day Typical simulated peak hour
total travel times, hours
125
105
170
80,000
75
75
140
70,000
55
60
75
60,000
Center turn
overpass
Echelon
interchange
Median
u-turn
Intersection
volume,
veh/day Center Turn Overpass
Disadvantages
High structure cost
Difficult to design if conventional-/' target='blank' class='doin' >streets are not
perpendicular
Visibility to businesses blocked by structure
Cost to obtain rights to design A Review of the Menu
Median u-turn
Bowtie
Superconventional-/' target='blank' class='doin' >street
Continuous flow
intersection
Single quadrant
Continuous green T
Echelon
Center turn overpass
Plus 4 others