roundabouts – operational performance and off ramp queue ... presentations/wednesday/71/li... ·...

Roundabouts –Operational Performance and Off Ramp

Queue Length

Zhixia (Richard) Li, Ph.D.

Assistant Professor, University of Louisville

&

Madhav Chitturi, Ph.D., Andrea Bill, David Noyce, Ph.D.

University of Wisconsin-Madison

Presentation Outline

• Problem Statement and Research Objectives

• Simulation Modeling

• Analysis Results

• Conclusions

2

• Problem Statement• Roundabouts have been

constructed at ramp terminals in some states to replace traditional signalized intersections.

• Potential issues with the operational performances arise and need to be studied:• Queue spillback from roundabout off-

ramps to the highway: arterial traffic is less impeded;

• Effect of heavy vehicles: larger critical gap and follow-up headway for trucks.

Problem Statement and Objectives Simulation Modeling

Analysis Results

Conclusions

3

Source: http://www.mtjengineering.com/project/ih-94-and-county-highway-tt/

• Research Objectives• Analyze the operational performance of roundabouts at ramp

terminals under different volume conditions, heavy vehicle percentages, and freeway lane closure statuses

• Compare the performances of roundabouts and signalized intersections at ramp terminals

• Develop guidance on choosing between roundabouts and signals at ramp terminals

• Develop signal metering warrant and strategies for congested roundabouts at ramp terminals.

4

Problem Statement and Objectives Simulation Modeling

Analysis Results

Conclusions

• Modeling roundabout interchange in VISSIM• Developed based on interchange at Hwy 53 and CTH O, in Oshkosh, Wisconsin

5

Simulation ModelingProblem Statement

and ObjectivesAnalysis Results Conclusions

• Modeling double-lane roundabout and signalized interchanges• Developed based on the single-lane roundabout interchange model

6



Characteristic Value

Off ramp length 2500 ft

Arterial approach length 5000 ft

Distance between roundabouts 700 ft / 400 ft

Arterial movement

distribution

Left turn 25% / 50% (heavy arterial left-turn)

Through 49% / 24% (heavy arterial left-turn)

Right turn 25%

U turn 1%

Off-ramp

movement

distribution

Left turn 50% / 75% (heavy off ramp left-turn)

Through 1%

Right turn 48% / 23% (heavy off ramp left-turn)

U turn 1%

Bypass right-turn lane both arterial and off-ramp entrance

Off ramp left- and right- turn storage length 400 ft

Arterial right-turn bypass lane storage length 320 ft

Traffic volume

distribution on

off ramp lanes

Left lane

left turn and U turn

(50% of all left turns, and

100% of all U turns)

Middle lane

left turn and through

(50% of all left turns, and

100% of all through movement)

Right laneright turn only

(100% of all right turns)

Traffic volume

distribution on

arterial lanes

Left lane

left turn, through, and U turn

(100% of all left turns,

25% of all through movement, and

100% of all U turns)

Middle lanethrough only

(75% of all through movement)

Right laneRight turn only

(100% of all right turns)

Freeway Speed 75 mph

Arterial Speed 45 mph

• Calibration of roundabout interchange simulation models• Calibration method

• Critical headway (tc) and follow-up headway (tf) as the surrogate measure of roundabout capacity.

• Calibration is to adjust VISSIM parameters (priority rules, car following model, reduced speed areas) to achieve simulated tc and tf equal to target tc and tf..

• Effect of heavy vehicle was considered in the calibration.7



( )cBv

pceC Ae

3600

( / 2)

3600

f

c f

At

t tB

• Calibration of the double-lane roundabout interchange simulation model• Target tc and tf

• Based on numbers suggested in Wisconsin FDM (for double-lane roundabouts)

• tc for cars = 4.0 s

• tf for cars = 2.8 s

• tc for heavy vehicles = 5.0 s

• tf for heavy vehicles = 3.8 s

8

Lane Calibrated Target

tc (s)

Simulated/

Calibrated tc

(s)

Targe

t tf (s)

Simulated/

Calibrated tf

(s)

WB*

Arterial

(East RAB)

Left LanePC* 4.0 3.97 (0.36)* 2.8 2.77 (0.67)

HV* 5.0 5.02 (0.63) 3.8 3.83 (0.52)

Middle LanePC 4.0 4.00 (0.27) 2.8 2.79 (0.66)

HV 5.0 5.04 (0.59) 3.8 3.82 (0.54)

Bypass Right-

turn Lane

PC 4.2 4.17 (0.33) 2.8 2.76 (0.51)

HV 5.2 5.22 (0.45) 3.8 3.79 (0.48)

EB*

Arterial

(West RAB)

Left LanePC 4.0 3.98 (0.45) 2.8 2.81 (0.71)

HV 5.0 4.95 (0.55) 3.8 3.84 (0.74)

Middle LanePC 4.0 4.04 (0.40) 2.8 2.80 (0.67)

HV 5.0 5.04 (0.63) 3.8 3.82 (0.55)

Bypass Right-

turn Lane

PC 4.2 4.22 (0.39) 2.8 2.78 (0.49)

HV 5.2 5.17 (0.48) 3.8 3.84 (0.58)

NB* Off Ramp

(East RAB)

Left LanePC 4.0 3.98 (0.37) 2.8 2.75 (0.61)

HV 5.0 5.00 (0.53) 3.8 3.84 (0.62)

Middle LanePC 4.0 4.00 (0.39) 2.8 2.78 (0.63)

HV 5.0 4.99 (0.50) 3.8 3.84 (0.69)

Bypass Right-

turn Lane

PC 4.2 4.17 (0.16) 2.8 2.76 (0.68)

HV 5.2 5.15 (0.50) 3.8 3.82 (1.07)

SB* Off Ramp

(West RAB)

Left LanePC 4.0 4.01 (0.34) 2.8 2.83 (0.61)

HV 5.0 4.95 (0.60) 3.8 3.83 (0.58)

Middle LanePC 4.0 4.04 (0.45) 2.8 2.84 (0.84)

HV 5.0 4.99 (0.46) 3.8 3.84 (0.70)

Bypass Right-

turn Lane

PC 4.2 4.15 (0.18) 2.8 2.78 (0.69)

HV 5.2 2.78 (0.69) 3.8 3.75 (1.20)

WB Interior

(West RAB)

Left LanePC 4.0 4.01 (0.39) 2.8 2.79 (0.64)

HV 5.0 5.04 (0.59) 3.8 3.84 (0.61)

Right LanePC 4.0 3.95 (0.44) 2.8 2.84 (0.59)

HV 5.0 4.98 (0.53) 3.8 3.81 (0.58)

PC 4.0 3.99 (0.38) 2.8 2.82 (0.62)



• Calibration of signalized interchange simulation models• Use the same VISSIM parameter values as used in the roundabout simulation

models.

• The gap acceptance behavior on all the bypass right-turn lanes was the same as that in the roundabout interchange simulation models.

• TTI phasing with single controller was used in the simulation model per the guidance in the Wisconsin Traffic Signal Design Manual (TSDM).

9



• Simulation Experiment Design• High traffic demand to cover extreme traffic conditions in peak

hours• Sum of entering plus circulating/conflicting demand ≤1500 vph for the

single-lane interchanges.

• Sum of entering plus circulating/conflicting demand ≤2550 vph for the double-lane interchanges.

• Asymmetric traffic demands at two intersections to be consistent with reality

Analysis ResultsProblem Statement and Objectives

Simulating Modeling

Conclusions

10

• Simulation Experiments• Experiment design for double-lane cases

• Totally, 1440 unique experiments for double-lane roundabout interchange

• Totally, 1440 unique experiments for double-lane signalized diamond interchange


Simulating Modeling

Conclusions

11

Arterial

HV%

Spacing

(ft)

Off Ramp

Left

Turn%

Arterial

Left

Turn%

Off Ramp

HV%

Westbound

Arterial

Demand

(vph)

Eastbound

Arterial

Demand

(vph)

Northbound

Off Ramp

Demand

(vph)

Southbound

Off Ramp

Demand

(vph)

5% 700 50% 25% 5% 100 200 200 100

400 75% 50% 10% 200 400 400 200

15% 300 600 600 300

20% 400 800 800 400

25% 500 1000 1000 500

600 1200 1200 600

• Simulation Experiment• Experiments for signalized interchanges: optimized signal timing

• Each experiment has its own optimized timing obtained using PASSER III.

• The research team initially explored methods to model the TTI phasing in Synchro.

• However, Synchro only supported ring-barrier phasing and was not appropriate for modeling TTI phasing.


Simulating Modeling

Conclusions

12

Performance Measures• Control delays and LOS: specified by O-D movements • Queue length: 95th percentile queue length for off ramps and arterial

approaches

Method for Operational Analysis• A generalized method to simplify the analysis and comparison between

roundabout and signalized diamond interchanges.• This method considered operational performance at (1) off ramp entrance

approach and (2) arterial entrance approach, rather than specific O-D movements.

• NB, SB, WB, and EB approaches were not considered separately, rather,• Data from NB and SB off ramps were integrated and both off ramps were

generalized as “off ramp approach”.• Data from EB and WB arterial approaches were combined and both

approaches were generalized as “arterial approach”. • Control delays for off ramp and arterial approaches are the performance

measures• Computed based on the control delays for specific O-D movements.


Simulating Modeling

Conclusions

13

Identification of Factors that Impact Control Delays• Linear regression analyses were performed to identify/confirm significant

factors.

• In the regression analysis, T test was used to measure significance of each potential factor involved in the regression analysis.


Simulating Modeling

Conclusions

14

Variables Tested

Effects on Off Ramp Control Delay at Double-Lane Interchanges

Roundabout Signal

p-value +/- Effect a p-value +/- Effect

Sum of Entering and Circulating/Conflicting Traffic Demands

0.0001* + 0.0001* +

Ramp Spacing 0.982 n/a 0.0001* +

Ramp Heavy Vehicle Percentage 0.0001* + 0.0001* +

* denotes statistically significant factors. P-value cut-off value = 0.05a. only applicable to statistically significant factors; positive effect indicates that increasing the value of the factor will increase the delay; vice versa for negative effect.

Identification of Factors that Impact Control Delays


Simulating Modeling

Conclusions

15

Variables Tested

Effects on Arterial Control Delay at Double-Lane Interchanges

Roundabout Signal

p-value +/- Effect a p-value +/- Effect


0.0001* + 0.0001* +

Ramp Spacing0.0001* + 0.0001* +

Ramp Heavy Vehicle Percentage0.0001* + 0.001* +

* denotes statistically significant factors. P-value cut-off value = 0.05a. only applicable to statistically significant factors; positive effect indicates that increasing the value of the factor will increase the delay; vice versa for negative effect.

Matrix Summarizing Factors that Impact Control Delays


Simulating Modeling

Conclusions

16

Off Ramp Delay Arterial Delay

Roundabout Signal Roundabout Signal

Double-LaneE+CHV%

E+CSPACING

HV%

E+CSPACING

HV%

E+CSPACING

HV%

E+C denotes sum of entering and circulating/conflict traffic demands; SPACING denotes ramp spacing;HV% denotes ramp heavy vehicle percentage.

Results for Double-Lane Interchanges

17

Off Ramp Control Delay: Comparison between Double-Lane Roundabout and Signalized Interchanges (Ramp Spacing = 400 ft)


Simulating Modeling

Conclusions

18



Simulating Modeling

Conclusions

19



Simulating Modeling

Conclusions

20



Simulating Modeling

Conclusions

21

Arterial Control Delay: Comparison between Double-Lane Roundabout and Signalized Interchanges (Ramp Spacing = 400 ft)


Simulating Modeling

Conclusions

22



Simulating Modeling

Conclusions

23



Simulating Modeling

Conclusions

24



Simulating Modeling

Conclusions

25

Selection Guidelines for Double-Lane Interchanges


Simulating Modeling

Conclusions

26

Vc+e (vph)

Select Signal Select Roundabout

Ramp Spacing

= 400 ft

Off Ramp Approach

HV% = 5% ≥2100 <2100

HV% = 10% ≥2010 <2010

HV% = 15% ≥1950 <1950

HV% = 20% ≥1900 <1900

HV% = 25% ≥1860 <1860

Arterial Approach All HV% None ≤2130

Ramp Spacing

= 700 ft

Off Ramp Approach

HV% = 5% ≥2100 <2100

HV% = 10% ≥2010 <2010

HV% = 15% ≥2080 <2080

HV% = 20% ≥2030 <2030

HV% = 25% ≥2000 <2000

Arterial Approach All HV% None ≤2130

Analysis of Queue Length at Roundabout Interchanges

27

Identification of Factors that Impact Queue Lengths at Roundabout Interchanges


Simulating Modeling

Conclusions

28

Variables Tested

Effects on Off Ramp Queue Length at Roundabout Interchanges

Single-Lane Double-Lanep-value +/- Effect a p-value +/- Effect a


0.0001* + 0.0001* +

Ramp Spacing 0.354 n/a 0.954 n/aRamp Heavy Vehicle Percentage 0.0001* + 0.001* +* denotes statistically significant factors. P-value cut-off value = 0.05a. only applicable to statistically significant factors; positive effect indicates that increasing the value of the factor will increase the queue length; vice versa for negative effect.

Matrix Summarizing Factors that Impact Queue Length at Roundabout Interchanges


Simulating Modeling

Conclusions

29

Off Ramp Queue Length

Single-Lane Roundabout Interchange

E+CHV%

Double-Lane Roundabout Interchange

E+CHV%

E+C denotes sum of entering and circulating traffic demands; and

HV% denotes ramp heavy vehicle percentage.

Off Ramp Queue Length for Single-Lane Interchanges


Simulating Modeling

Conclusions

30

(a) (b)

(c) (d)

(e)

Figure 88 Regression Analyses of Off Ramp Queue Length for Single-Lane Roundabout

Interchanges (Vc+e ≤ 1160): (a) Ramp HV% = 5%; (b) Ramp HV% = 10%; (c) Ramp HV%

= 15%; (d) Ramp HV% = 20%; (e) Ramp HV% = 25%

y = 6.07122e0.00315x

R² = 0.85057

0

50

100

150

200

250

300

350

400

0 200 400 600 800 1000 1200 1400

Qu

eu

e L

en

gth

(ft

)

Entering Demand Plus Circulating Demand (vph)

Single-Lane Roundabout InterchangeOff Ramp Queue Length vs Traffic Demand

[Ramp Spacing = 400, 700 ft; Ramp HV% = 0.05][Entering Demand + Circulating Demand ≤ 1160]

y = 6.40229e0.00322x

R² = 0.84992

0

50

100

150

200

250

300

350

400

450

0 200 400 600 800 1000 1200 1400

Qu

eu

e L

en

gth

(ft

)




y = 6.67214e0.00329x

R² = 0.84614

0

100

200

300

400

500

600

0 200 400 600 800 1000 1200 1400

Qu

eu

e L

en

gth

(ft

)




y = 6.86914e0.00335x

R² = 0.84462

0

100

200

300

400

500

600

700

0 200 400 600 800 1000 1200 1400

Qu

eu

e L

en

gth

(ft

)




y = 7.20580e0.00340x

R² = 0.83843

0

100

200

300

400

500

600

700

800

0 200 400 600 800 1000 1200 1400

Qu

eu

e L

en

gth

(ft

)




Off Ramp Queue Length for Double-Lane Roundabout Interchange


Simulating Modeling

Conclusions

31

(a) (b)

(c) (d)

(e)

Figure 96 Regression Analyses of Off Ramp Queue Length for Double-Lane Roundabout

Interchanges (Vc+e ≤ 2103): (a) Ramp HV% = 5%; (b) Ramp HV% = 10%; (c) Ramp HV%

= 15%; (d) Ramp HV% = 20%; (e) Ramp HV% = 25%

y = 8.22104e0.00157x

R² = 0.92690

0

50

100

150

200

250

300

350

0 500 1000 1500 2000 2500

Qu

eu

e L

en

gth

(ft

)


Double-Lane Roundabout InterchangeOff Ramp Queue Length vs Traffic Demand


y = 8.43942e0.00162x

R² = 0.93265

0

50

100

150

200

250

300

350

400

450

0 500 1000 1500 2000 2500

Qu

eu

e L

en

gth

(ft

)




y = 8.54390e0.00168x

R² = 0.92571

0

100

200

300

400

500

600

0 500 1000 1500 2000 2500

Qu

eu

e L

en

gth

(ft

)




y = 8.85648e0.00170x

R² = 0.91977

0

100

200

300

400

500

600

700

800

0 500 1000 1500 2000 2500

Qu

eu

e L

en

gth

(ft

)




y = 8.87719e0.00175x

R² = 0.91644

0

100

200

300

400

500

600

700

800

900

0 500 1000 1500 2000 2500

Qu

eu

e L

en

gth

(ft

)




Queue Length Look-up Table: Off Rampof Single-Lane Roundabout Interchange


Simulating Modeling

Conclusions

32

Single-Lane #

400, 700 ft #

5% - 25% HV #

Vc+e a (vph) Queue Length

Off Ramp

Approach

200-1160

≥1161 Unstable state: increases rapidly and can easily spill back to freeway mainline

regardless of available ramp storage length.

a. Vc+e denotes sum of entering and circulating demands.

# Indicates applicable single/double lane, ramp spacing, and off ramp heavy vehicle percentage for the look-up table.

0

50

100

150

200

250

300

350

400

0 200 400 600 800 1000 1200 1400

Qu

eu

e L

en

gth

(ft

)

Entering Demand (Ve) Plus Circulating (Vc) Demand (vph)

Single-Lane InterchangeOff Ramp Queue Length vs Traffic Demand

[Ramp Spacing = 400, 700 ft][200 vph ≤ Ve+c ≤ 1160 vph]

HV = 5%

HV = 10%

HV = 15%

HV = 20%

HV = 25%

Queue Length Look-up Table: Off Ramp of Double-Lane Roundabout Interchange


Simulating Modeling

Conclusions

33

Double-Lane #

400, 700 ft #

5% - 25% HV #

Vc+e a (vph) Queue Length

Off Ramp

Approach

400-2103

≥2104 Unstable state: increases rapidly and can easily spill back to freeway mainline

regardless of available ramp storage length.

a. Vc+e denotes sum of entering and circulating demands.

# Indicates applicable single/double lane, ramp spacing, and off ramp heavy vehicle percentage for the look-up table.

0

50

100

150

200

250

300

350

400

0 500 1000 1500 2000 2500Q

ueu

e Le

ngt

h (

ft)

Entering Demand (Ve) Plus Circulating Demand (Vc) (vph)

Double-Lane InterchangeOff Ramp Queue Length vs Traffic Demand

[Ramp Spacing = 400, 700 ft][400 vph ≤ Ve+c ≤ 2103 vph]

HV = 5%

HV = 10%

HV = 15%

HV = 20%

HV = 25%

Warrant for Signal Metering at Roundabout Interchanges• According to NCHRP Report 672, roundabout metering is typically used

when the subject entrance experiences dominatingly high traffic volume during peak periods.

• According analysis results summarized in Chapter 4, a roundabout entrance may experience bad LOS not only when the subject entrance has high traffic volume but also when the circulating traffic at the entrance is substantially high.

• Results in Chapter 4 of the report indicate:• A roundabout entrance may have LOS F occasionally when Vc+e reaches a

certain threshold (Threshold A);

• A roundabout entrance has LOS F consistently when Vc+e reaches a higher threshold (Threshold B).


Simulating Modeling

Conclusions

34

400, 700 ft #

5% - 25% HV #Threshold Type Vc+e

a Thresholds of LOS F (vph)

Single-Lane Interchange

Threshold A 1180

Threshold B 1450

Double-Lane Interchange

Threshold A 2130

Threshold B 2270

a. Vc+e denotes sum of entering and circulating demands at the off ramp entrance.# Indicates applicable ramp spacing, and off ramp heavy vehicle percentage for the look-up table.

3-Step Warrant for Signal Metering at Roundabout Interchanges• Step 1: Make sure the queue on the off ramp will not spill back to

the freeway mainline.• What to do: Check off-ramp queue length look-up tables identify at what Vc+e

(Threshold C), the off ramp queue length will exceed the available off ramp storage length.

• Step 2: Mare sure the off ramp will not fail. There are two alternative strategies to choose:• Strategy 1: For conservative operational planning, signal metering must be

warranted when Vc+e at the off ramp roundabout entrance reaches Threshold A. This strategy ensures that the LOS of the off ramp will never fall under F.

• Strategy 2: For more aggressive operational planning, signal metering will not be warranted until Vc+e at the off ramp roundabout entrance reaches Threshold B. This strategy intrigues the signal metering only when there is no possibility for the off ramp to work at an LOS other than F.

• What to do: Choose one of the aforementioned strategies. Once strategy is chosen, check Table 32 to identify the corresponding Threshold A/B.

• Step 3: The threshold to warrant signal metering at the subject roundabout interchange is determined by the following equation:


Simulating Modeling

Conclusions

35

( , / )Threshold Min Threshold C Threshold A B

Conclusions

• Key interchange type selection guidelines are summarized as follows

ConclusionsProblem Statement and Objectives

Simulation Modeling

Analysis Results

36

Single-Lane

Off Ramp Approach

Roundabout interchange is recommended when Vc+e* is below a

certain threshold ranging between 1100 vph and 1200 vph depending

upon the off ramp heavy vehicle percentages.

Arterial Approach

Roundabout interchange is recommended when Vc+e is below 1140

vph for the “Ve – Vc > 100 vph” case; and

Roundabout interchange is recommended when Vc+e is below 1290

vph for the “Ve – Vc ≤ 100 vph” case.

Double-Lane

Off Ramp Approach

Roundabout interchange is recommended LOS when Vc+e is below a

certain threshold ranging between 2100 vph and 1860 vph depending

upon the off ramp heavy vehicle percentages and ramp spacing.

Arterial Approach

Roundabout interchange is recommended under any conditions.

* Vc+e denotes sum of entering demand and circulating/conflicting demand;* Ve denotes entering demand; and* Vc denotes circulating/conflicting demand.

Conclusions

• Selection guidelines can assist transportation professionals in determining whether signalized and roundabout interchanges are the more appropriate interchange type.

• Once roundabout interchange is chosen, queue length look-up tables can assist in estimating queue length under various conditions, which in turn help determine the required off ramp and arterial storage length to prevent queue spill back to the freeway mainline or to the adjacent upstream intersection on the arterial.

• The three-step procedure to determine the traffic demand threshold that warrants signal metering can assist transportation professionals to identify scenarios when signal metering is required.

ConclusionsProblem Statement and Objectives

Simulation Modeling

Analysis Results

37

The project was sponsored by Wisconsin Department of Transportation.

Thank You !

QUESTIONS?

roundabouts – operational performance and off ramp queue ... presentations/wednesday/71/li... ·...

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