efficient intersection operations by reducing conflict points: … · 2019-06-11 · efficient...
TRANSCRIPT
Efficient Intersection Operations by Reducing Conflict Points: Offset, Grade-Separated, and Other Intersection Ideas Joe Hummer, PhD, PE State Traffic Management Engineer
For RPO Conference, July, 2018
Wider Intersections Don’t Always Solve the Problem
2
Alternative Intersections are Often Part of the Solution
3
Alternative Design Principles 1. Minimize conflict points 2. Separate conflict points 3. Favor through movement 4. Use half signals 5. Minimize signal phases
4
Conflict Points Crossing Merging Diverging
Full Signals Need to Be Properly Spaced
• For 40% efficiency at 45 mph: Cycle, sec Simultaneous
sweet spot, ft Alternate
sweet spot, ft
60 0 – 400 1580 – 2380
80 0 – 530 2110 – 3170
100 0 – 660 2640 – 3960
120 0 – 790 3170 – 4750
150 0 - 990 3960 - 5940
Fewer Phases Better
More arrows = More phases = Less time for Main Through Movement
100%
"1" phase
100%
"1" phase (no signal)
70%
30%
2 Phase
70%
30%
2 Phase
50%
30%
3 Phase
% Green Time
Available for Main Through
Movement
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
30%
30%
20%
"8" Phase
20%
30%
30%
20%
"8" Phase
20% Time that Main Through Movement sees red and cannot go
Time that Main Through Movement sees green and can go
20%
You Know About Some Alternative Intersections
7
Here Are Some Newer Ideas • Offset intersections • Seven-phase signals • Three-legged continuous flow intersections
(CFIs) • Grade-separated intersections
• They all follow the design principles • The best results may be from mixing and
matching 8
A Common Problem: Someone Wants to Add a
Fourth Leg
9
A Fourth Leg Means Violating all Five Principles
1. Minimize conflict points--Higher 2. Separate conflict points--Concentrated 3. Favor through movement--Robbed 4. Use half signals—Full signal 5. Minimize signal phases—Four critical phases
10
Options to Consider • Deny request for fourth leg • Right-in and right-out • Superstreet • Median u-turn
Should also consider: • Offset intersection • Seven-phase signal
11
These could work, but are often strongly resisted
Create an Offset Intersection • Tech Assistance Report from ITRE, 2017 • Optimum spacing 600 feet or so • Reduce conflict points from 32 to 18 • Reduce signal phases to three critical
– Keep cycle shorter • Great with superstreet design
12
Offset Intersection Safety
• Mixed results in literature – Small study from VA found offset intersections
safer – International literature review found offset
intersections generally safer – Couple studies showed no significant
difference – One study in rural area showed offset
associated with more severe crashes 13
Offset Intersection Efficiency
• Travel times overall decreased with: – High major street through volumes
• Travel times overall increased with: – High left turns volumes
• Travel times almost always reduced for major street movements
14
Offset Good for Pedestrians Too
15
Seven-Phase Signal
• Eliminate phase 8 – Where 3+4 < 7 – And 8 can be
redirected well – No ped crossing of
east leg • Stays as three
critical phases
16
There are Similar Intersections in Place in the US
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Example US-64 at NC-345, Manteo
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Current v/c = 0.63
Example Seven-Phase Signal
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Estimated v/c = 0.41
Mix and Match: Seven-Phase Signal and Quadrant Intersection
20
Four-Legged CFIs • Pros
– Highest capacity of any at-grade intersection
– No extra distance traveled – ROW not large
• Cons – No solid data on safety yet – No access to frontages
with ramps – Poor service for
pedestrians 21
Four-Stage Pedestrian Crossing
1 2
3
4
Three-Legged CFI Overcomes the Ped and Access Limitations
• Ramp on main street
• Ramp on minor street
23
Mix and Match: CFI and Superstreet
24
Mix and Match: CFI and Median U-Turn
25
Grade-Separated Intersection
• Intersections with at least one bridge • Neither intersecting route is a freeway
– Both streets can have signals • Higher cost than at-grade • Higher capacity that at-grade • Best designs follow five rules
What is Wrong with This Design in This Spot?
• Downtown Raleigh
What is Wrong with This Design in This Spot?
Poor ped. service
Poor progression
No metering
High speed
Too much ROW Too much frontage
Existing Design Single Quadrant, Durham
New Design Echelon Interchange
New Design Echelon Interchange
New Design Center Turn Overpass
Arterial
Arterial or collector
New Design Center Turn Overpass
Arterial
Arterial or collector
New Design Two-Level Signalized
New Design Two-Level Signalized
New Design Half Single-Point and Half Superstreet
From Dennis Eyler, SRF Consulting, Minneapolis, MN, 2011
New Design Double Contraflow
Developed by Hummer, 2012
Scoring the Designs Design Good aspects Poor aspects Total score
(of 60)
Parclo A Capacity, conflict points Progression, distance traveled, peds
31
Diamond Distance traveled, unusual maneuvers
Progression, capacity, peds 32
Cloverleaf Capacity, progression, wrong way potential
Distance traveled, cost, peds
23
Single quadrant Metering, cost, peds Efficiency, unusual maneuvers
40
Echelon Everything except… Bridge cost, peds, patent 47
Center turn Everything except… Bridge cost, patent 46
Two level Everything except… Bridge cost, patent 49
Half-half Everything Nothing 49
Contraflow Everything Nothing 49 38
Rescue a Diamond
39
Convert a Cloverleaf
NC-54 Raleigh Rd.
US-15/501 Fordham Blvd.
Summary
• Follow the five rules • Use alternatives designs • Newer variations are available
– Offset intersections – Seven-phase signals – Three-legged CFIs
• Mixing and matching has big advantages
41
Summary
• Don’t use an interchange in place of a grade-separated intersection – At least examine all alternatives – Don’t sacrifice progression, metering, peds,
ROW, frontage,… – Many creative possibilities – Research underway
