Network-wide traffic congestion models:
Application to street network design and
left-turn treatments
Transportation Engineering and Safety Conference
December 8, 2016
Vikash V. Gayah
The Pennsylvania State University
Traffic congestion forms locally at locations where traffic
demand exceeds available capacity for movement…
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On isolated facilities, we can reduce congestion by
increasing flow of vehicle through congested areas…
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However, these local level metrics might not be as useful
when tackling questions about large network operations
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Instead, we need to adopt measures and models that
characterize traffic behavior across the entire network…
Penn State has been working on developing these models/metrics and applying them to
help better understand existing problems
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One timely question that requires network-wide
thinking: use of one-way vs. two-way streets
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Traffic engineers generally prefer one-way streets
because of intersection simplicity…
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One-way streets
Two-phase signal timing plan
Two-way streets
Four-phase signal timing plan
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…however, one-way streets force vehicles to travel
longer distances to reach their destination
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Pertinent question becomes…how do
we rectify these competing impacts?
Network-wide measures can be used to combine
both of these factors into one metric
• Trip completion rate [veh/hr]
– Measure of how quickly vehicles are able to reach
their destinations and exit the network
– Better measure of network purpose
Supply of available movement [veh-mi/hr]
Demand for each trip [mi]
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Maximum flow for each network type can be
estimated based on intersection layout
Two-way streets with single lane
approaches
Two-way streets with left-turn
pockets
One-way streets
Two-way streets with nonoverlapping left turn lanes
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Two-way streets with
overlapping left turn lanes
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Probability theory and geometry can be used to estimate
additional travel distance on general OW networks
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Putting this information together allows us to compare
(relative) performance of different networks…
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Potential solution: two-way streets with prohibited
left turns offer high flows of one-way networks…
Two-way streets with prohibited
left turns
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Four-phase signal timing plan
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…and have fewer restrictions (so vehicles have to
travel shorter distances than one-way networks)
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The shorter distances hold on average for all origindestination pairs in a network
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Two-way street networks with prohibited left turns
outperforms other networks in terms of capacity
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Networks do not always operate at capacity. Times exists in
which prohibiting left turns would be detrimental…
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Analytical tools or simulation can be used to quantify trip
completion rates for range of network accumulations
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NEF - 15x15 network, 250 m blocks
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Trip completion rate [trips/hr]
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0
0
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Accumulation [veh]
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10
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Comparison of trip completion rates confirm prohibiting
left turns reduces efficiency in light/heavy congestion
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NEF - 15x15 network, 250 m blocks
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LT Permitted (with LT pocket)
Trip completion rate [trips/hr]
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LT prohibited
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2
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0
0
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Accumulation [veh]
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Dynamic strategy can help provide a “best of both
worlds”…
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NEF - 15x15 network, 250 m blocks
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LT Permitted (with LT pocket)
LT Prohibited
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Trip completion rate [trips/hr]
Dyn LT Permitted (with LT pocket)
Dyn LT Prohibited
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2
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0
0
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Accumulation [veh]
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Many municipalities do this on an ad-hoc fashion or at
individual locations but can have network-wide benefits
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Concluding remarks…
• Network-wide measures and models can help to
answer “big-picture” questions that are difficult
to answer using traditional metrics
– Focus more on overall goal of transportation network
as opposed to local behaviors
• Example applied to street network layout and
treatment of left turns
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Concluding remarks…
• One-way streets offer higher flows but longer
travel distances
– Trip completion rate used to combine two competing
effects
• Findings:
– Two-way networks offer higher capacities when trips
are short
– One-way networks offer higher capacities when trips
are long
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Concluding remarks…
• Two-way street networks with prohibited left
turns offer highest capacity for all trip lengths
• Trip completion rates are generally lower when
left turns are prohibited in very light or heavy
congestion
– Dynamic strategy could prohibit left turns during
certain times/traffic conditions to maximize network
productivity
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Thank you!
Vikash V. Gayah
Assistant Professor
Department of Civil and Environmental Engineering
The Pennsylvania State University
231L Sackett Building
University Park PA 16802
[email protected]
phone: 814-865-4014
http://www.engr.psu.edu/gayah
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