The penetration of a car windshield by a flying piece of plywood looked horrific when presented in several photographs by the Ontario Provincial Police (OPP) on their Twitter account on January 2, 2019. The incident occurred on Hwy 410 just northeast of Toronto, Ontario. The OPP photos are shown below.

In actuality the passengers were not “lucky to be alive” as claimed on the OPP Twitter account. The piece of wood has come to rest while still embedded inĀ  the windshield. This means that “just before” being embedded it was travelling very slowly. Progressing slightly further back in time it was moving faster with respect to the vehicle but it was also not penetrating into the interior as much as when it was embedded at final rest. So, no, in this specific incident the occupants were not “lucky to be alive” but experienced the reported minor injuries that would be expected.

On the other hand, the OPP Twitter posting raised an important point about the danger of more serious incidents. While you are travelling at 110 km/h in one direction a piece of debris from an opposing vehicle may be travelling at 110 km/h in the opposing direction. That is a closing speed of 220 km/h. That debris would be coming at you at about 61 metres every second. If you consider a reaction time of about 2 seconds to initiate a response and then consider that your vehicle cannot steer away or brake “on a dime”, you are very unlikely to avoid a piece of flying debris like this unless you have a great deal of prior warning. In fact in almost all incidents drivers cannot initiate a successful evasive action and avoiding such an impact is very much a case of luck.

Even a piece of debris that weighs very little (i.e. is of a small mass) becomes dangerous because of the speed (velocity in the opposite direction toward you) at which it is closing toward you. So even very small objects, when they are travelling very quickly toward you can be lethal. Broad objects like a piece of plywood may be less dangerous when they strike you with the broad, flat surface. But turning that plywood around 90 degrees so that the edge is coming at you will make a world of difference with respect to your injury and survival. No seat-belts or air bags will save you.

Modern windshields are actually quite resistant to penetration as they are formed of two pieces of tempered glass with a thick plastic layer in between. It is that plastic layer that keeps the windshield intact even though the glass may become completely destroyed. In severe head impacts during severe frontal collisions, unrestrained occupants have been known to tear that plastic layer to some degree but complete head penetration through that layer would be very rare. It is those small but solid pieces of debris (something like the size of a bowling ball) that can be extremely dangerous when they contain ragged edges that can pierce through the plastic and have enough mass to apply a considerable “point load” to a small part of the windshield. Obviously anything more massive increases the danger.

Considering the danger of possible debris being projected from one side of a divided highway onto the other is would be advisable to consider driving in the right lane of such a multi-lane highway when such a choice is possible. While these incidents are rare, they are difficult to avoid and can be lethal.