## Sunday, 19 July 2015

The New Zealand Herald reports:
Motorists are driving in excess of 125 km/h on the new Tauranga Eastern Link road and police, who fear someone could be killed, say the road is "not a racetrack".
The stretch of State Highway 2 between Te Maunga and Domain Rd opened in May and since then motorists have been caught driving at excessive speeds.
While the construction was underway the stretch of SH3 was narrow and has speed restrictions, Senior Sergeant Ian Campion said...
On Monday four motorists were caught driving in excess of 125 km/h and ranged from a male learner driver, through to an experienced female driver in her 40s. Police would not be disclose the precise speeds or the top speed.
The highway is due to be completed shortly with the opening of the remainder of the road, and police are encouraging everyone who uses the road to make safety their number one priority.
This is yet another example of the Peltzman effect (which I've written about in the context of driving here and here) - if driving is made safer, people will drive faster. The basic explanation goes like this: Rational (or quasi-rational) drivers will weigh up the costs and benefits of driving faster. The benefits include less time wasted on the roads (an opportunity cost - you give up some time you could spend doing something else). Moreover, the marginal benefits probably decrease the more a driver speeds (because opportunity costs increase the more time is wasted). The costs of driving faster include an increased risk of a serious car accident - this cost is made up of two parts: (1) the probability of a serious accident occurring; and (2) the health and other costs of the accident itself. The marginal costs increase as speed increases, because the probability of an accident and its seriousness both increase.

If they are optimising, the driver will choose to drive at the speed where the marginal benefit (MB) of driving faster is exactly equal to the marginal cost (MC0). This occurs at S0 in the diagram below. At this point, driving a little bit faster entails a higher additional cost than the benefit they would receive (which is why they will drive no faster than S0).

When driving is made safer (such as by replacing a narrow road with a wide and straight multi-lane highway like the Tauranga Eastern Link road), this changes the incentives that drivers face. The cost of driving fast falls (since the chance of being involved in a head-on collision on the bridge is reduced). So, in the diagram above, marginal costs of speed are lower (MC1). This increases the optimal driving speed to S1. What we would observe then is drivers driving faster because of the perceptions of greater safety.

So, the road may be safer, but the driving behaviour changes to offset some of that additional safety. That isn't to say that safety isn't the drivers' number one priority - only that they are weighing up increased safety against the benefits of additional speed.

Previously on my blog: