Post on 22-Dec-2021
Optimal Highway Traffic Control using a
Velocity-Cell Transmission Model
Alessandro Castagnotto
Nicholas Wong
Outline
Problem Description and Motivation
Cell Transmission Model
Godunov Flux function
Cell Transmission Model for Velocity
Optimal Control Problem
Flux as a minimization problem
Conclusions and Outlook
Problem Description and Motivation
Optimal ramp metering for highways using a
velocity based model
Density data is relatively difficult to measure
Proliferation of GPS and accelerometer
equipped smart phones makes velocity data of
vehicles on road available in abundance
Cell Transmission Model
Discrete approximation to the LWR PDE
βΟ π₯, π‘
βt+βQ π π₯, π‘
βx=0
Highway is divided into discrete cells (indexed by i)
Density of vehicles in a cell, Ο changes in relation to the flows in and out, G the Godunov flux function
ππ π + 1 = ππ π +πΊπ π β πΊπ+1 π
Has been shown to be equivalent for piecewise affine flux
Godunov Flux Function
Our flux function is the Greenshieldβs Flux
Previous work shows that a velocity model will not be
equivalent unless the relationship between velocity and
density is affine
Using the relationship π = π£π we obtain the following
fundamental diagrams
Cell Transmission Model for Velocity
Combining above equations give the following
velocity based model
π£ππ+1 = π πβ1 π£π
π +βπ
βπ₯πΊ π£πβ1
π , π£ππ β πΊ π£π
π, π£π+1π
Godunov Flux
The Godunov flux is given by:
πππ = πΊ π£π
π, π£π+1π =
π π£π+1 , ππ π£π β₯ π£π+1 β₯ π£ππ π£π , ππ π£π+1 β₯ π£π β₯ π£ππ π£π , ππ π£π+1 β₯ π£π β₯ π£π
πππ π π£π , π (π£π+1) , ππ π£π β₯ π£π+1
Optimal Control Problem
Minimize TTT
s.t. π£π0, π = 1, β¦ , πΌ
πΊ π£β1, π£0 = πΊ 0
πΊ π£πΌ, π£πΌ+1 = πΊ πΌ
πΊ π£π , π£π+1 = πΊπππ’πππ£ πππ’π₯
This leads to a problem as our flux is governed by a
set of if conditions
Flux as a Minimisation Problem
An equivalent formulation for the flux can be found by:
πΊ π£π , π£π+1 = ππππ ππ π£π β π£π+1 π(π£π)
π ππ π£π β π£π+1 π(π£π+1)ππmax
π = β1,π€βππ π£π β€ π£π β€ π£π+1β₯ 1, πππ πππ ππ‘βππ πππ ππ
Flux as a Minimisation Problem
Mixed integer non-linear program
Minimize π½πΊ
Subject to π½πΊ = πΌ β 2π + π1 + π2
πΌ β₯ π
πΌ β₯ βπ
π β€ π ππ π£1 β π£2 π π£1
π β€ π ππ π£1 β π£2 π(π£2)
π β€ 3 β π1 β π2 ππππ₯
π1 β₯ 2π ππ π£2 β π£π
π1 β₯ 0
π2 β₯ 2π ππ π£2 β π£1
π2 β₯ 0
Conclusions and Outlook
We are able to correctly determine flux despite non
linearities in the problem
Slack variables affect our cost
For control purposes the cost formulation needs to
be tweaked or to reformulate the problem such that
slack variables are not required (such as Big-M
formulation)
Infeasibilities with solvers when using system
dynamics
Godunov Flux
π£π β€ π£π+1 β€ π£πΆ βΉ πΊπ = π π£2
Current and next cell are
both congested
Cell i+1 is moving faster
The flux in cell i can be
equal to that in cell i+1
Godunov Flux
π£π β€ π£π β€ π£π+1 βΉ πΊπ = π π£π
Cell i is congested
Cell i+1 is not
Cell i can discharge at
maximum rate
Godunov Flux
π£π β€ π£π β€ π£π+1 βΉ πΊπ = π π£1
Both cells are
uncongested
All vehicles can flow
from i to i+1
Godunov Flux
π£π+1 β€ π£π βΉ πΊπ = πππ π π£1 , π π£2
Cell i+1 is moving slower
than cell I
The amount that can
flow into cell i+1 is upper
bounded by the flux of
cars outside cell i+1