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Effectiveness of a co-operative traffic control > July 12, 2009 > slide 1Institute of Transportation Systems > Aerospace technology for road and railway

On the effectiveness of a co-operative traffic control

Peter WagnerInstitute of Transportation SystemsGerman Aerospace Centre

Institute of Transportation Systems > Aerospace technology for road and railwayEffectiveness of a co-operative traffic control > 12. Juli 2009 > slide 2

Background

Recent project “co-operative traffic control“(financed by German car manufacturer Volkswagen): Some vehicles can communicate with the signal controllerBut what should they tell it? Within this project: vehicles act as “virtual loop detector”......data are the input into a traditional vehicle-actuated control Two main results (by micro-simulation, here: VISSIM):

Not surprising: gain of vehicle-actuated control depend on penetration rate


Delays

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Background

Recent project “co-operative traffic control“(financed by German car manufacturer Volkswagen): Some vehicles can communicate with the signal controllerBut what should they tell it? Within this project: vehicles act as “virtual loop detector”......data are the input into a traditional vehicle-actuated control Two main results (by micro-simulation, here: VISSIM):

Not surprising: gain of vehicle-actuated control depend on penetration rate Surprising: even with 100% penetration rate, the overall gain is small compared with a (optimal!) fixed-cycle controller


What can be achieved by a vehicle-actuated control?

Restrict to a simple VA recipe, i.e. the following logic:when current green > minimum greenand current headway > critical headway (unit extension)go to next phase

(BTW: when go to next phase anyway)Nothing more complicated like MOVA, OPAC, RODIN,...Measure of performance: delay time !(difference between shortest and actual travel time)Use the simplest possible set-up to avoid most of the dirty details


Geometry / Set-up

In the following, we used a simple four arm intersection, SymmetricIn the following: two arms:No turnsOf course, there is a so calledsaturation flow

and something that measures “randomness”

(I=0: deterministic, I=1: Poisson)

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Further details:

Ideal car-following model (roughly Gipps)Poisson source (better: an approximation to it, no good microscopic model allows distances zero Erlang distribution)Signal is either red or green (not yellow)Compare VA with optimum fixed cycle (FC) for each demand

Compute gain as function of

Note: results still not independent of implementation details (currently, we check against VISSIM)


Newell, Theory of Highway Traffic Signals

gain by VA can be bigger than 100%! (symmetric saturated demand, , L: total loss time of signal)

Is this realistic?

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But: why does it work?

delay vs timefixed cycle control changesphase regularly...

while vehicle-actuatedcontrol...

does this on demand


Vehicle-actuated control: more details

two observations1. outflow during green2. delay vs green

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Vehicle-actuated control: more details (cont’d)

two observations1. outflow during green2. delay vs green

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Vehicle-actuated control: why does it work at all? (3)

Critical ingredience: fluctuations while queue is cleared; they originate from:

Input (Poisson source) Or: “internal“ – distributed preferred headways, vehicle lengths, reaction-times all add noise

and: what is the optimal green? Too big better than to small, since d increases fast for small G (once created, queue stays forever)VA tries to estimate the moment where no vehicle is hampered any more delay is zero on this approach go to next phaseIt works only in the presence of noise!


Map of improvements

“Bug“: G becomes to short; introduction of yellow already cures that (one more vehicle can pass), but better to use a better equation for the optimal green

In fact, about 60% can be gained compared to an optimal FCAt this point, FC has reached already saturation


Cut along


Similar plot, different model

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Towards a co-operative control

What should the vehicle tell the traffic signal controller?Just the headway at a certain loop detector positions not sufficientBetter idea: use the delay times itself! Each vehicle knows it, and can report it to the signal controllerwhich tries to minimize the delays (increase green if delay is too bad)preliminary results: works better than the original idea of using the headwaysbut worse than a fine tuned vehicle-actuated control

Much work left to be done here!


Conclusions (preliminary)

VA works because traffic is stochasticMechanism is the fluctuating queue length, which is estimated and utilized by the VAVA minimizes delayGain is small (in this simple form) given a small or asymmetric demandWhen demand is symmetric, gains up to 60% are possible(less than Newell predicts, but who expected this to be right?)Saturation point of the signal goes to larger demand valuesOften, the signal is co-ordinated with neighboring ones, thus reducing the gain further.

Effectiveness of a co-operative traffic control > July 12, 2009 > slide 18Institute of Transportation Systems > Aerospace technology for road and railway

Thank you for your attention!


A simple consideration

n vehicles standing at the red arm, one vehicle approaching at the green arm; is it better to stop it or not?okay, let’s do the math. Let \ell the distance where the vehicle is detected, vmax it’s speed, and \Lambda the total loss time upon switching the signal to the next phase. Obviously two cases:

let it pass: then only the n stopped vehicles add to the objective function (delay d):

if we stop the vehicle, the total delay is:

we have to compare the two. Note, that ideally R = n/Q + 2\Lambda.


Drawings...

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On the effectiveness of a co-operative traffic control�BackgroundDelaysBackgroundWhat can be achieved by a vehicle-actuated control? Geometry / Set-upFurther details:Newell, Theory of Highway Traffic Signals�But: why does it work?Vehicle-actuated control: more detailsVehicle-actuated control: more details (cont’d)Vehicle-actuated control: why does it work at all? (3)Map of improvementsCut alongSimilar plot, different modelTowards a co-operative controlConclusions (preliminary)Thank you for your attention! A simple considerationDrawings...Drawings...

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