Returning for another guest post, James Glover is once again drawn to a beer coaster for some quick, if somewhat morbid, calculations. For those taking to the road over the Christmas period, this post should also serve as a reminder to drive carefully!

Why did the chicken cross the road? To get to The Other Side. This twist on the ending to the iconic joke was based on the observation of a single dead chicken on the road while I was returning by car from Sydney to Melbourne at the end of my recent touring holiday. The holiday in fact started two weeks earlier when I was driving around Tasmania. While there were no chickens, there was a dead sperm whale on the road.

In Tasmania I noted that in addition to its abundance of quality food, beer and wines, it has a remarkable supply of one other thing compared to the mainland: native fauna road kill. If you think that the occasional dead kangaroo (or more likely fox) you see driving in the country is plenty, then you haven’t been to Tassie. We are talking a dead possum or wallaby every kilometre or so, which means if you are driving at 100km/hr means you see one every 36 seconds. While it is unfortunate, particularly for the animals involved, to see so many native animals dead (but no foxes because they have been eradicated from the island), it is actually cause for joy because it indicates a very healthy population of native wildlife.

This got me thinking. Could you actually use the number of road kill to estimate the density of animals living in the bush? The answer is yes, and without any derivation or proof I present it here:

`density = road kill ratio/kill zone area`

where

```road kill ratio = av. distance between cars/
kill zone area = 2 x car velocity x time x car width```

## 26 thoughts on “Roadkill Arithmetic”

1. Magpie

Hey Zebra!

I’ll guess you didn’t go to the West Coast…

Next time you’ve got to do some hiking around the Pieman River and talk to the locals. I hear they are fond of hunting, so they could tell you about the local fauna. Maybe, you know, you could find a Tassie Tiger.

2. Zebra

Dear Magpie – I’m sure you know the Magpie poem – “one is for sorrow, two is for joy…”

always good to hear from you…have a happy xmas mate.

3. Zebra

I sent this separately to the Mule in case he wanted to put it up which is a derivation of the above formula. In case you are interested here it is:

I assume the animals are evenly spread out with density r. A car travels along a (wlog straight) path for time T so road length L=vT where v is is the velocity. It sweeps out an area of w*v*t where w is the width of the car. The number of cars of the road is 2*L/dc (2 because there are 2 lanes) where dc is the average distance between cars so the the total number of dead animals after time T is N=r*(2*w*L)*(L/dc). If db is the average distance between dead animals then N=L/db so substituting this in and rearranging gives: r=(dc/db)/(2*w*v*T) = road kill ratio/kill zone area.

One aspect of this formula is that it implicitly assumes the animals aren’t moving. Does this matter? Well it is based on an “expected number” formula so in a sense the animal density is smeared (pardon the pun) over all possible states so no it doesn’t. There are a number of possible reasons why it might not be accurate the most obvious one is that animals may avoid the road or be attracted to it. I was told that Tammar Wallabies are “particularly stupid” and they certainly seem to be well represented amongst the road kill. If one had an alternative estimate for bush density of a particular species then you could use it to work out to what extent a particular species was likely to end up road kill given their population, and attraction or repulsion to roads. I guess that would be an interesting statistic in itself.

4. Emmjay

Thank Christ you don’t work for the climate change denialists (do you ?) or the poker machine lobby.

With maths like this, I reckon poker machine odds look pretty reasonable. And Alan Moran over at the ABC’s Drum is starting to make sense… Whoa !!!!

Have a great Christmas, sport. Drop into the Pig’s Arms some time.

5. PFH007

Very good!

Yes those wallabies are particularly committed to crossing to the other side (in both senses) at the most inopportune times.

But the possums are not far behind.

A few years ago we were driving at night (at about 15kph) and spotted a Brushtail possum walking down the dividing line away from us. As we approached to within about 20 metres , he glanced overs his haunches but then kept on walking. We literally drove up beside him, waved, and then continued on.

I suppose he may have been reasonably safe provided he stayed right on the dividing line but I wouldn’t recommend it as a means of extending one’s membership of the gene pool.

6. Zebra

@PFH007 – in Tassie I had to stop to let an echidna cross the highway in front of me, which entailed traffic backing up behind me. This made me feel inordinately good all day for some reason. I recently read that echidnas split, in evolutionary terms, about 25m years ago from platypus. Given that monotremes have been around for 200m years it goes to show how resilient this last remnant of that unorthodox mammal group are. It is therefore even more surprising that we haven’t wiped them out in the last 200 years.

7. Occassional traveller

On my travels a few years ago I too noted the large amount of road kill vis the mainland – in particular in the south east.

As for the formula – I suspect that the mainland marsupials have pre-empted it and have found a way to avoid not only the night traffic but also your formula ….

:)

Have a great 2011

8. Dan

No leg pulling. Google “aerial fauna bridge NSW” and you’ll find them in approvals and env impact assessments up and down the highway. And the Hume too.

I’d love to know if the animals actually use them? Gotta be someone somewhere monitoring them. Assuming they weren’t just a sop to local greens.

9. Pfh007

They probably will use them judging from the possum highway in our street – the power lines are their preferred cat avoidance technique.

10. zebra

I love this but when they say on their website they are used by “possums, gliders and other animals” I wonder what “other animals”? Tree snakes?

11. Pfh007

Thanks Dan, but the possum survival rate seems pretty good near me. They seem to have already mastered abseiling, pole vaulting, trapeze work and from the sounds i hear , cage fighting.

12. zebra

@dan – so it’s not enough dropbears can attack innocent bushwalkers but now with these devices they can drop in on cyclists and cars as well.

13. Pfh007

@zebra – wise advice re drop bears! Apparently you are safe in a Prius – it seems they acknowledge good intentions and move on to more deserving prey – hummer drivers perhaps.

14. Zebra

@Dan – I ‘ve heard in the lower north shore of Sydney that dropbears target open topped Porsche convertibles. They’re attracted to the authentic plush leather interiors apparently.

15. Plunko

Errrr – Stubby, the only reason a whale would cross the road would be if he/she was getting fat from bad puns (this would be over-krill) and was looking for a whale weigh station….sorry, couldn’t help myself!!!

16. dan

Sorry to come back to this. (We’ve all moved on to the JJJ Top 100) but, just back from 4 glorious days in Tasmania – including driving much of the east coast from Hobart to Launceston the long way round. You simply could not imagine the amount of road kill. Wallabies, Possums, Devils, Foxes, Cats, Birds, no doubt the odd Wombat – it was hard to tell (still no whales). So I have to concur with original post.

According to this Catalyst report: “The density of roadkill in Tasmania – about one dead animal every three kilometres – is unprecedented worldwide.” (http://www.abc.net.au/catalyst/stories/3198257.htm)

Unprecedented Worldwide!

Certainly seemed that way to me. And it was very sad.

17. Emmjay

Our last road trip around Tasmania was about seven or eight years ago. The roadkill was pretty horrific then. So were the monster timber tucks raging around those tiny blacktop strips at something close to Mach II. Co-incidence ?

18. zebra

@dan – another excellent link. I may try and get in touch with the CSIRO guy.
@emmajay – I was warned about the timber trucks but found very few of those or RVs which I was also warned about. Except the RV that came around the corner on the wrong side and noticed me at the last minute and swerved. Now I know how roadkill feel. Or at least road-near-miss.