This explorable features an agent based model for road traffic and congestion. The model captures a phenomenon that most of us have witnessed on highways: phantom traffic jams, also known as traffic shocks or ghost jams. These are spontaneously emergent congested segments that move slowly and oppositely to the traffic. The explorable illustrates that phantom jams are more likely to occur if the variability in car speeds is higher:
So, if say 90% of the cars try moving at 120 km/h and 10 percent at 150 km/h, everyone might end up going 80 km/h on average. Whereas if everyone travelled at about 120 km/h no reduction of collective traffic flow occurs.
This is the best demonstration I’ve ever seen of the creation of phantom traffic jams. Playing with the (interactive) model, you can set up scenarios and watch how they affect traffic throughput. When everybody drives at 120km/h, everything’s fine. But when everybody drives at between 120km/h and 150km/h, traffic jams occur which result in everybody having to slow down to less than 120km/h!
This counterintuitive fact is hard to explain to people, but this interactive model makes it perhaps a little bit easier.
(There are, of course, other – more human – factors that result in an increased frequency of phantom traffic jams, but mathematicians are rarely concerned with what happens in the real world!)
When we woke up this morning Oxford was caked with a blanket of snow, about two inches thick and growing fast. Ruth, JTA and I thought that we’d make the most of it and go for a walk along the Cherwell, and by the time we were heading back the snow was ankle-deep. Reaching the corner of the street where we live we helped a few stranded motorists whose vehicles had taken one look at the hill near our house and said “fuck this for a lark.” Specifically, we helped them by pushing their cars off junctions and out of the way of other cars. It didn’t take long to realise that the chaos that was the series of junctions on the main road was only getting worse, and, caught out by our own sense of social conscience (and perhaps at least a little inspired by a recent story we’d read), we decided that we could be doing more.
We trekked back to Earth and collected hardy boots, hi-visibility jackets, shovels, and brushes, and made our way back to the junction. And, for the next hour or two, we worked at clearing the road and rescuing motorists. Before long there were others coming out of their houses and workplaces and helping: pushing cars up hills and clearing snow and ice from troublesome parts of the road. Highlights included:
Rescuing dozens of motorists who’d otherwise have been completely stuck.
Shoveling clear an escape road for vehicles that couldn’t make it up the hill.
Giving directions to motorists whose routes were blocked, to pedestrians whose buses had been cancelled, etc.
Stopping all traffic in order to prioritise ambulances, as we’re on a hospital approach road. You’d be amazed how many motorists will do what you tell them when you’re wearing a flourescent jacket.
Getting thanked by a great number of people.
Getting complaints from a minority of people who were angry that we were shovelling and not salting/gritting: presumably they thought that we were employed by the council.
Meeting like-minded helpful people who came out of their houses and workplaces to lend a hand.
We returned to Earth and drank mulled wine with Hanna, a woman who lives up the road from us who came out and helped. She’d been expecting her boyfriend (who’s visiting for the weekend) but he’s among the thousands of people stuck out in the snow, and even five hours after he was expected he hadn’t yet arrived. Then we made snow angels in the garden.
And because karma doesn’t believe in us, the universe repaid our kindness by having our boiler break down again (but in a different way) this evening. So now we’re sat in blankets in the living room.
Aside from the main point of this article, I found it most amusing to find that state-run Indian elephants are fitted with reflectors ‘in an attempt to prevent road accidents’. What does it take to accidentally crash into an elephant?
Another fantastic story from the BBC: this one took place yesterday, and so I missed it, as I didn’t come in to work. Apparently this lorry full of cheese caught fire on the A44, on my usual route to the office. The driver said: “I saw the fire starting but by the time I’d gone back to the cab to get the fire extinguisher the whole lot had started to go on fire.”
1. Cheese burns?
2. What route did he take back to the cab? Via Bow Street?
3. How does combusion occur in the hold of a moving lorry full of milk produce?