Traf­fic jams are or­gan­ised chaos

Weekend Herald - - Science & Tech - with Her­ald sci­ence writer Jamie Mor­ton: @jamien­zher­ald

It won’t seem like it to those of us stuck in hol­i­day grid­lock this sum­mer, but there is a sci­ence to traf­fic jams.

Sci­en­tists have used sta­tis­ti­cal physics to hunt for dis­cern­able pat­terns in six cities, the near­est be­ing Mel­bourne, in a study that seems to have pin­pointed the mo­ment at which traf­fic backs up.

The re­searchers com­pared in­di­vid­ual driv­ers’ travel times and how long it took them to reach their des­ti­na­tions from the be­gin­ning of peak hour, with driv­ers start­ing their jour­neys over the next hour of that peak pe­riod.

The time it took driv­ers to reach their des­ti­na­tions was la­belled re­cov­ery time.

Team mem­ber Dr Meead Saberi, of the Univer­sity of New South Wales, said as re­cov­ery time passed the crit­i­cal thresh­old where cars us­ing the net­work out­weighed the net­work’s full ca­pac­ity, traf­fic started to move be­yond con­ges­tion to net­work col­lapse, or grid­lock.

“We have found that this sim­ple re­cov­ery time mea­sure is di­rectly re­lated to de­mand and sup­ply — no sur­prise,” Saberi said.

“What is sur­pris­ing is that all the cities that we have stud­ied per­form sim­i­larly.”

In other words, de­spite the dif­fer­ences be­tween cities in to­pog­ra­phy, pop­u­la­tion size, in­fra­struc­ture, de­mand and other char­ac­ter­is­tics par­tic­u­lar to each city tran­si­tion to grid­lock hap­pened in ev­ery city in a sim­i­lar fash­ion.

The point when this tran­si­tion hap­pens might be unique to each city, but the re­searchers now have a quan­tifi­able mea­sure for it.

“The de­mand over sup­ply ra­tio that we have mea­sured is the ra­tio of the ve­hi­cle kilo­me­tres trav­elled in a city to the to­tal ve­hi­cle dis­tance the road net­work can sup­port per hour,” Saberi said.

“When this ra­tio ex­ceeds a crit­i­cal value, we see tran­si­tion to grid­lock. For ex­am­ple, a ma­jor global city like London may have a smaller crit­i­cal value and that’s why it sees grid­lock more of­ten than, say, a smaller city like Ade­laide.”

Trans­port au­thor­i­ties and govern­ments could use the find­ings to un­der­stand when and how traf­fic forms and how likely it could de­velop into net­work col­lapse.

Mon­i­tor­ing the num­ber of ve­hi­cles en­ter­ing the net­work and the re­cov­ery time could pro­vide an early in­di­ca­tion of whether a grid­lock is likely to hap­pen or not. “This in­for­ma­tion can be used to in­ter­vene in the net­work by man­ag­ing travel de­mand or in­creas­ing trans­port sup­ply when and where needed.”

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