By John Pickering 26/10/2018

Dear Driver,

When you backed out of a driveway and did not even see how I swerved around behind your car to avoid T-boning you, how dare you have the temerity to tell me you were careful!  I was 7 feet tall, dressed in bright yellow and traveling at no more than 10 km/h.  Perhaps a simple lesson in physics will help you and your fellow “driveway backers” to realise how dangerous you are and to adopt safer driving practices.

In the diagram you can see a car backing out of a driveway.  Typically when you are at the edge of your property and have a fence (see photo below) blocking your view of the footpath you are able to see about 1.7 metres along the footpath.  Let us imagine that there is a child on a trike riding at 5 km/h just out of your line of sight.  How long  does it take them to travel that 1.67 metres?  The physics is quite easy.

5 km/h is 5000 metres in 60 x 60 seconds, ie about 1.4 m/s.  Putting this in the formula above means that it takes about 1.2 seconds for the child to travel that 1.67 metres.

Now consider this. According to design guidelines for safe bicycle use 2.5 seconds must be allowed for someone to observe the danger, react, apply brakes and stop.  In other words, if you covered the distance from your driveway to the middle of the footpath, about 1 metre, in under 1.2 seconds you will almost certainly hit the child.  That is a speed of just 3 km/h!!!!!

Now consider who else is on the footpath, all legally:

• Pedestrians 5 km/h
• Joggers 5- 15 km/h
• Kids on skateboards or scooters 10 km/h
• Child on bicycle with small wheels, 10 km/h
• Mobility scooter, 5-10 km/h
• Me on my Trikke, 10 km/h
• Postie on a bike 5-10 km/h.

For those going 10 km/h your speed needs to be just over 1.5 km/h to hit someone! That’s the legal people … but the Lime scooters at 20 km/h mean it all the more necessary to slow down.

So, before you do some damage here is what you can do:

• Never back out of a driveway unless you really really must.  If you think you must because of the design of your driveway, change the design!
• Cut back those hedges, remove some of that fence so that you can see further. [ City councils… please make a by-law to make this happen].
• Always always always stop at the end of your driveway (BEFORE THE FOOTPATH) and toot a horn.  Then proceed very very slowly.

By the way, you are legally obliged to give way:

(1) A driver entering or exiting a driveway must give way to a road user on a footpath, cycle path, or shared path (as described by clause 11.1A(1)).

Thank you for considering the physics of maiming a child, may you never find your self in such a terrible situation.

Regards,

Dr John Pickering

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Feature Image: Intangible Arts https://www.flickr.com/photos/intangible/ under Creative Commons Attribution 2.0 licence.

### 0 Responses to “The physics of maiming a child (repost because of “those” scooters)”

• Frank Williams says:

Thank you for this article. A well thought out analysis of how easy it is to do this and how easy it is to avoid a disaster.

It’s definitely time to get out there and trim the hedge and negotiate with the neighbor to reduce the height of the boundary fence ( at least near the footpath ).

• John Pickering says:

Hello Frank, I didn’t pick up on your comment until now. Thank’s very much – it is nice to know that I’ve had some kind of impact. All then best, John.