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Aimsun Saturation Flow Rate Calibration for Hybrid SimulationChallenges and Recommendations
John Bennett and Brian Betts
August, 2017
Traffic modelling levels
Three broad levels
Macro:
- Large scale, strategic, no
detailed representation of
congestion.
Meso:
- Medium scale, models
intersections in detail, capable
of macro and dynamic
assignment.
Micro:
- Finest level of detail, complex
signal operation, models
individual vehicle movements.
Aimsun capable of all three
levels and “Hybrid”
simulation
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Source: Barcelo, J. Casas J, Garcia D & Perarnau J
(2005)
Aimsun meso assignment
Discrete event simulation:
- Simulation time changes when
an event occurs.
- Vehicle generation, vehicle
node movement, traffic signal
change.
- Simplified car-following and gap
acceptance model
Vehicle considered only as
it enters and exits a node
section.
Does not consider
acceleration/ deceleration
or details lane changing
behaviour.
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Source: TSS Aimsun 8.1.4 User’s Manual 2016
Challenges for Hybrid simulation
Differences in vehicle
movement, car-following
and gap acceptance.
No acceleration/
deceleration or detailed lane
changing.
Requirement for a level of
comparability between
meso and micro area
operation.
Intersection throughput/
saturation flow
comparability is key factor.
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Source: TSS Aimsun 8.1.4 User’s Manual 2016
What is saturation flow?
“The maximum uniform
discharge rate across a stop
line” (Webster F.V. and Cobbe
B.M. 1963).
Can be measured on-site
(ideal) or predicted using
TRRL and HCM2010
formulas.
Research indicates that
saturation flow is primarily
impacted by geometrical
factors:
- Lane widths
- Gradient
- Turn radii
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Source: HCM2010
Saturation flow impact variables
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Source: TRRL and HCM2010
Saturation flow adjustment by location
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Site designation
Description of characteristics Adjustment to TRRL RR67
Good Dual carriageway intersections.
No noticeable interference from pedestrians, parked vehicles, right –turning traffic (either owing to their absence of because special provision is made for them).
Good visibility and adequately large turning radii.
Exit of adequate width and alignment.
Good quality road surface.
+10%
Average Some characteristics of good sites and poor sites. -5% to 10%
Poor Average speeds low.
Some interference from standing vehicle, pedestrians and right turning traffic.
Poor visibility and or poor alignment of intersection.
Busy shopping street with pedestrian activity.
Poor road surface.
Traffic calming measures on either/both entry and exit.
Congestion or downstream queueing discouraging drivers from pulling away cleanly.
15% to 25%
Source: TRRL Food for Thought Article 97 (2005)
Saturation flow calibration in Aimsun
Vehicle reaction times (Reaction Time and Reaction Time at Stop
parameters).
Vehicle kinematics in micro simulation only (acceleration/ deceleration
profiles, etc.)
Speeds on turns – common approach used to adjust saturation flows in
VISSIM.
Section Jam Density in meso simulation only:
- Governs the number of vehicles that can stay at the same time in a section
(model link).
TRRL and HCM2010 variables such as lane width and gradient have no
significant impact on saturation flow.
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Analysis aim and objectives
Challenges for Hybrid simulation:
- Calibration of saturation flow rate using non-traditional impact variables.
- Disconnect between micro and meso vehicle movement.
Aim:
- To calibrate saturation flow and achieve operational comparability between meso and
micro simulation within the same network.
Objectives:
- Identify parameters that have most impact on saturation flow in micro and meso.
- Determine parameters that result in typical saturation flow rates in micro and meso
simulation, which could be used as a starting point in a model calibration process.
- Recommend parameters that result in comparable intersection operation in micro and
meso, thereby providing a level of comparability for hybrid simulation.
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Analysis methodology
Development of a dummy signalised approach using Aimsun version 8.1.4.
Assessment steps:
1. Calculate saturation flow rate in micro using software default reaction time and
turn speed values.
2. Refine reaction time and turn speed parameters in micro to achieve typical
saturation flow of around 2,000 vehicles per hour.
3. Calculate average delay for the intersection approach using preferred reaction
time and turn speed parameters.
4. Calculate average delay for the intersection approach using meso simulation with
software default reaction time and turn speed parameters.
5. Refine reaction time, turn speed and section jam density parameters produce
approach average delay comparable to levels achieved in micro.
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Analysis assumptions
Single lane intersection approach to eliminate impacts of vehicle lane changing.
Assignment of car vehicle type only.
Consistent signal timings and operation.
Consistent approach volume and vehicle arrival profile.
Calibration of saturation flow rates were assessed separately for each of these
adjustment methods, not in combination.
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Analysis results
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Micro simulation parametersMethod Turn
speed (kph)
Time step (s)
Reaction time (s)
Reaction time at stop (s)
Saturation flow rate
(vph)
Average delay (s)
Software default parameters
70 0.8 0.8 1.2 2,571 26.21
Preferred turn speed
25 0.8 0.8 1.2 2,057 27.84
Preferred global reaction times
70 0.5 1.0 1.6 2,057 30.50
Preferred section reaction times
70 0.5 1.0 1.6 2,000 31.61
Meso simulation parametersMethod Turn
speed (kph)
Jam density
(veh/km)
Reaction time (s)
Reaction time at stop (s)
Saturation flow rate
(vph)
Average delay (s)
Software default parameters
70 200 1.2 1.6 N/A 14.59
Preferred turn speed
25 200 1.2 1.6 N/A 14.59
Preferred section jam density
70 40 1.2 1.6 N/A 31.41
Preferred global reaction times
70 200 2.1 1.6 N/A 32.16
Preferred section reaction times
70 200 2.04 1.6 N/A 32.53
Other calibration considerations
Vehicle lane changing on approach to stop line:
- Site specific, can be managed using Look Ahead Distance parameters and solid lines.
Impacts of calibration of angled turn movements
Lane utilisation:
- Appears balanced in micro by default, but appears to favour kerb-side lanes in meso.
Impact of posted speeds, lane widths and gradient
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Conclusions
Saturation flow rate is not directly influenced in Aimsun by the same parameters
outlined by TRRL and HCM2010.
Software default parameters (version 8.1.4) don’t appear to produce typical saturation
flow rates.
Software default parameters (version 8.1.4) don’t appear to produce a level of
comparability between meso and micro assignment in terms of saturation flow.
Reaction times and turn speeds can be used to influence saturation flow in micro:
- Recommended values to be used as a starting point for calibration.
Turn speed does not seem to impact saturation flow in meso.
Reaction times and jam density can be used to influence saturation flow in meso and
calibrate to micro delay output:
- Recommended approach and parameters values for starting point in calibration.
Review other factors such as lane changing and lane utilisation:
- Meso appears to weight lane utilisation to kerb side lanes.
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References
Akcelik R. (1981), Traffic Signals: Capacity and Timing Analysis: ARRB Research
Record 123, Australian Road Research Board.
Barcelo, J, Casas J, Garcia D & Perarnau J (2005), Methodological Notes on
Combining Macr, Meso and Micro Models for Transportation Analysis.
Gipps, P.G. (1981), A behavioural car-following model for computer simulation.
Transportation Research Board Part B, 15, 105-111.
Gipps, P.G. (1986a), A model for the structure of lane-changing decisions.
Transportation Research - B. Vol. 20-B, No. 5, pp. 403-414.
Gipps, P.G. (1986b). MULTSIM: A Model for Simulating Vehicular Traffic on Multi-Lane
Arterial Roads. Mathematics and Computers in Simulation, 28. 291-295.
Highway Capacity Manual 2010: Volume 3 – Interrupted Flow: Chapter 18 Signalized
Intersections, Transport Research Board.
Kimber RM, Mcdonald M, & Hounsell NB (1986), The prediction of saturation flows for
single road junctions controlled by traffic signals, Research Report 67, Transport and
Road Research Laboratory.
Law, Averill M. and Kelton W. David, (1991) Simulation Modeling and Analysis.
McGraw-Hill International Editions. Second Edition.
Transport System Solutions (2016) Aimsun version 8.1.4 User’s Manual.
Webster F.V., Cobbe B.M., (1966) Traffic Signals, Note 34, Transport and Road
Research Laboratory.
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