Sunday, August 06, 2006

Speed, Speeding, or Relative Speed? Huh?!

While motorists are constantly under threat of being caught speeding, this cyclist is interested in a different kind of speed.

The Theory

Let's say a car turns a bend and gets first look at a cyclist in front, who is 50 meters down the road.

If the car is travelling at 30mph, and the bike is stationary (0 mph). The car will be where the bike is, in 3.728 seconds.

If the car is travelling at 30mph, and the bike is travelling at 10 mph. The car will be where the bike is, in 5.592 seconds.

If the car is travelling at 30mph, and the bike is travelling at 25 mph. The car will be where the bike is, in 22.369 seconds.

This example makes it patently clear that the faster the cyclist's speed, the longer the car behind has; to spot the cyclist, consider the best way to overtake, to plan and execute this manoeuvre.

With the '30mph car' vs '25mph bike' example, the ample time gap (22 secs before overtake) is present, because the car is only catching the bike at a rate of (30mph - 25mph = ) 5mph.
This is the relative speed between both vehicles.


Should the car be breaking the 30mph speed limit, let's say 45 mph (which is 198 feet per second!), and the bike is going uphill at 10mph, the car will be on top of the bike in 3.196 seconds.

That is quicker than if the bike was even stationary, and the car was doing 30mph!
(Relative speed in these examples:
e.g.1: 45mph - 10mph = 35mph.
e.g. 2: 30mph - 0mph = 30mph)

There is no doubt modern cars have fantastic technology, and in dry theoretical conditions can brake very efficiently once the pedal is applied. However, the weakest link as ever, is the human decision maker at the helm.

3 seconds sounds ok while sitting at a PC reading this, but tasks like mirror checking, indicating, changing gear, turning, checking speed, turning, etc, all take valuable seconds.

The more closely matched a bike and car's speeds are (i.e. a low relative speed), the more time the driver has to carry out the right checks and tasks, to overtake the cyclist safely. Cars being driven at excess speed have the opposite effect, so is more dangerous.

It is worth mentioning of course that managing relative speed (between bicycle and car) is largely the responsibility of the motorised vehicle, because it is much easier to control speed up or down - via engine power, than it is via pedal power.

The Faster The Better

Perfection sometimes, is when relative speed = 0 mph. I feel very safe bombing down hill at 25mph in the middle of a lane, at a similar speed to surrounding traffic. Because I just become another vehicle, in line. Well away from parked car door-zones, and in easy view of other drivers.

A small fan-club might form behind, of drivers who aren't able to overtake me, but they're not at all bothered, because we're all making fair progress.

Of course there are many other circumstances, which will require a different approach, and a relevant speed. (E.g. Filtering slowly past traffic queues / Densely populated pedestrian areas / etc).

Situations like heavy traffic might look daunting to some, but if the traffic is only ambling along at 10-15mph or so, this makes it a lot easier for me on a bicycle join in at 15mph also, to achieve that relative speed of zero.

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