For a phantomtrafficjamtoform, theremustbe a lotofcarsontheroad.
Thatdoesn't meantherearenecessarilytoomanycarstopassthrough a stretchofroadwaysmoothly, atleastnotifeverydrivermaintainsthesameconsistentspeedandspacingfromotherdrivers.
Inthisdensebutflowingtraffic, itonlytakes a minordisturbancetosetoffthechainofeventsthatcauses a trafficjam.
Sayonedriverbrakesslightly.
Eachsuccessivedriverthenbrakes a littlemorestrongly, creating a waveofbrakelightsthatpropagatesbackwardthroughthecarsontheroad.
Thesestop-and-gowavescantravelalong a highwayformiles.
With a lowdensityofcarsontheroad, trafficflowssmoothlybecausesmalldisturbances, likeindividualcarschanginglanesorslowingdownat a curve, areabsorbedbyotherdrivers' adjustments.
Butoncethenumberofcarsontheroadexceeds a criticaldensity, generallywhencarsarespacedlessthan 35 metersapart, thesystem's behaviorchangesdramatically.
Abovethecriticaldensity, anyadditionalvehiclereducesthenumberofcarspersecondpassingthrough a givenpointontheroad.
Thisinturnmeansittakeslongerfor a localpileuptomoveoutof a sectionoftheroad, increasingvehicledensityevenmore, whicheventuallyaddsuptostop-and-gotraffic.
Driverstendnottorealizetheyneedtobrakefarinadvanceof a trafficjam, whichmeanstheyenduphavingtobrakehardertoavoid a collision.
Thesevehicleswouldmaintain a uniformspeed, safetypermitting, thatmatchestheaveragespeedoftheoverallflow, preventingtrafficwavesfromforming.
Insituationswherethere's already a trafficwave, theautomatedvehiclewouldbeabletoanticipateit, brakingsoonerandmoregraduallythan a humandriverandreducingthestrengthofthewave.
Anditwouldn't takethatmanyself-drivingcars.
In a recentexperiment, oneautonomousvehicleforevery 20 humandriverswasenoughtodampenandpreventtrafficwaves.
Trafficjamsarenotonly a dailyannoyance, they're a majorcauseoffatalities, wastedresources, andplanet-threateningpollution.