Riding Skills 101

When the Two Second Rule is not enough

The original version of this particular article almost dates to the earliest days of the internet, because I wrote it following a discussion on the Go Ride Forum on CompuServe - that should serve to date it accurately enough. But well over twenty years later, whenever I'm out driving a car, running training courses or simply watching other riders, one recurring error is following the vehicle ahead far too close. And whilst motorcyclists worry about being tailgated themselves, a surprising number of collisions happen when it's the rider running into the back of the vehicle ahead. It seems some rider errors never change.

The Highway Code says sensibly enough that we "should ride at a speed that allows us to stop in the distance we can see to be clear, and at a distance that will allow us to stop if the vehicle in front stops suddenly". It then goes onto quote some following distances and braking distances.

The trouble is that it doesn't really matter whether those distances are in feet, metres or car lengths, our biggest problem is trying to visualise these distances. It's tough to think just how far 75 feet or 23 metres or 6 car lengths at a standstill, let alone on the move at 30 mph. This is clear when performing demonstration  emergency stops. I'll often get the trainee to make where they THINK I'll stop if I hit the brakes at 30 mph. That usually puts them on the back foot, and it's rare to find anyone who moves confidently to mark their chosen spot.

Not uncommonly, I have to remind them of the 23 metre stopping distance. And then they often stand about half that distance away.
And that may be a clue as to why riders follow too close on the road. If we can't judge a short distance like 23 metres to within 50% at a standstill, what chance to they have on the road? (The way I judge it is that it's just over the length of a cricket pitch.)

But there's a solution, also in the Highway Code. It's the Two Second Rule. We watch the vehicle in front as it passes a fixed object (a lamppost, a tree, a shadow or even a seam in the road) and start talking: "only a fool breaks the Two Second Rule". In theory, that takes about two seconds to say, so if we have passed this point before we've finished talking, we're too close. If we get it all out without gabbling, we're far enough back. And in the wet, we double the Two Second Rule to four seconds.

So does it work? Whilst it's likely to be adequate around town, there's also some bad news that neither the Highway Code nor any of the other manuals or riding tips has made clear:

• the Two Second Rule is not a target - it's a MINIMUM safe distance and should be extended at all times whenever traffic conditions allow
• the stopping distances are based on a rider braking AS SOON AS the vehicle ahead slows - even an alert rider can be caught by SURPRISE! and that is well-known to delay our response
• stopping distance does not increase in a straight line with speed - if we double our speed, we QUADRUPLE stopping distance

It's the last point I want to focus on because it means there is a cross-over point beyond which we cannot stop even with a two second gap. My old buddy Steve Kelly got his trusty calculator out and we fed some data in. We travel 13 metres or 44 feet each sec at 30mph. Stopping (reaction and braking) distance at 30mph is quoted in the Highway Code as 23 metres or 75 feet. A modern motorcycle on decent tyres can brake much harder than the Highway Code allows. So we made two initial assumptions:

• an alert rider can react in around 0.5 second
• braking hard, a rider can achieve around 0.9G of braking force (more on that in a moment)

And we ignored wind resistance, poor road surfaces and all rolling friction. So here are the braking figures:

At 15mph

• In 2 seconds at constant speed you will travel   44.0ft
• Your stopping and reaction distance is             19.4ft
• Your margin for error is                                   24.6ft
• Your impact speed is                                       n/a

At 30 mph

• In 2 seconds at constant speed you will travel   88.0ft
• Your stopping and reaction distance is             55.4ft
• Your margin for error is                                 32.6ft
• Your impact speed is                                       n/a

At 45 mph

• In 2 seconds at constant speed you will travel 132.0ft
• Your stopping and reaction distance is             108.0ft
• Your margin for error is                                 23.8ft
• Your impact speed is                                       n/a

At 60 mph

• In 2 seconds at constant speed you will travel   176ft
• Your stopping and reaction distance is             178ft
• Your margin for error is                                 -2.0ft
• Your impact speed is                                       0.6mph

At 75mph

• In 2 seconds at constant speed you will travel   220ft
• Your stopping and reaction distance is             264ft
• Your margin for error is                                 -43.9ft
• Your impact speed is                                       12.5mph

At 90mph

• In 2 seconds at constant speed you will travel   264ft
• Your stopping and reaction distance is             367ft
• Your margin for error is                                 -103ft
• Your impact speed is                                       25.2mph

At 105mph

• In 2 seconds at constant speed you will travel   308ft
• Your stopping and reaction distance is             486ft
• Your margin for error is                                 -178ft
• Your impact speed is                                       38.5mph

I think you can see what is happening - at 60mph, maintaining a 2 second gap, you will crash at walking pace even if you are wide awake and brake as quickly as possible. At 90 (not an unusual motorway speed, you will have a serious accident with a big impact. (And at 150mph, you would hit the brickwall at a frightening and certainly fatal 80.2mph!)

What if we're caught by SURPRISE! and our reactions are a bit slow? If we double the reaction time to a not-unlikely one second (more on that in a moment too), here are the figures.

At 15mph

• In 2 seconds at constant speed you will travel   44.0ft
• Your stopping and reaction distance is               30.4ft
• Your margin for error is                                   13.6ft
• Your impact speed is                                         n/a

At 30 mph

• In 2 seconds at constant speed you will travel   88.0ft
• Your stopping and reaction distance is               77.4ft
• Your margin for error is                                   10.6ft
• Your impact speed is                                         n/a

At 45 mph

• In 2 seconds at constant speed you will travel   132.0ft
• Your stopping and reaction distance is             141.0ft
• Your margin for error is                                    -9.2ft
• Your impact speed is                                         2.9mph

At 60 mph

• In 2 seconds at constant speed you will travel   176ft
• Your stopping and reaction distance is               222ft
• Your margin for error is                                   -45.7ft
• Your impact speed is                                       12.4mph

At 75mph

• In 2 seconds at constant speed you will travel   220ft
• Your stopping and reaction distance is               319ft
• Your margin for error is                                   -98.9ft
• Your impact speed is                                       23.3mph

At 90mph

• In 2 seconds at constant speed you will travel   264ft
• Your stopping and reaction distance is               367ft
• Your margin for error is                                 -169ft
• Your impact speed is                                         35.1mph

At 105mph

• In 2 seconds at constant speed you will travel   308ft
• Your stopping and reaction distance is               563ft
• Your margin for error is                                  -255ft
• Your impact speed is                                         47.6mph

The important point is that now the crossover point is now 45mph. 

Not surprisingly some objections were raised during discussions.

"Rider over-reaction to a perceived emergency is a greater hazard in many situations than the actual emergency itself". I can't argue with this having done that myself and fallen off on several occasions, and it's why ABS is compulsory on new bikes in the EU. 

"It is well known that a modern motorcycle with good tyres can stop from 60 mph in under 110 feet; this is comparable to only a few automobiles such as Porsche, Corvette, Ferrari, etc. In the vast majority of situations where maximum braking is needed, the bike will be able to come to an absolute stop, well before the standard car or truck." A biking myth. It may just possibly have been true in the past, but with four wheels on the ground and ABS to boot (literally), I'm far from convinced that even a well-handled bike will out-stop a car. Few riders practice emergency stops and even with ABS too, braking a motorcycle at the absolute limit requires a degree of skill and machine control that a car driver simply doesn't need. 

"I don't think the Two Second Rule results in such dire consequences as the calculations imply. One is assuming a fixed object towards which one is braking. In reality, emergency braking is initiated in response to a vehicle ahead, also braking. That vehicle must decelerate from its velocity just as you do, so the Two Second Rule simply provides adequate reaction time to initiate your braking in response to the emergency ahead." It's another fine theory that as speeds rise and traffic spreads out, we can see and react to vehicles braking further ahead, but theory isn't always right. I can think of three incidents in front of me:

1. on a motorway, the car ahead of me drifted to the right, dipped its right front wheel in the gravel drain at the edge of the outside lane and was swung straight into the Armco barrier in the centre of the motorway. It immediately rolled over a couple of times and slid to a halt on the roof in front of me. That stopped pretty quick!
2. my brother was driving with me in the passenger seat when the car in front of us colliding head-on with a vehicle coming the opposite way that had drifted into our lane. That blocked the road ahead almost instantly. 
3. I was following a car when its offside suspension collapsed, and it spun 180 degrees and stopped dead facing me.

THESE are the situations the Two Second Rule is designed to rescue us from. 

And here's something else to think about. A study of emergency braking revealed that instead of our assumed 0.9G of deceleration, a typical rider is more likely to generate 0.6G. I've haven't repeated the calculations but I'm sure you realise that wil add SIGNIFICANTLY to all the stopping distances. 

We also originally banked on a reaction time of an alert rider of 0.5 of a second and double that for a rider caught by surprise. In fact, 0.5s is right at the top limit for reaction time. A more typical figure is around 0.7 sec for an alert driver. But even our 1s reaction time turns out to be much to optimistic in a real emergency. When we're taken by SURPRISE! our reaction time can become even longer. Two, even three seconds is common. It's almost certainly this delayed response that explains why many urban riders don't avoid junction collisions even when the accident investigation says they should. It wasn't that they were speeding (the more usual conclusion) but that they simply failed to react to the emergency because they were taken by SURPRISE!

As I've said before when discussing my advanced motorcycle training courses, that brings me to the need to PLAN for things to GO WRONG. We must be pragmatic if we are to have genuine Survival Skills. 
So if you've religiously adhered to the Two Second Rule till now, you may be a bit concerned to find it doesn't work as we go faster. Even though I knew stopping distances quadruple as speed doubles, I was a bit startled to discover the crossover point was so low. As Steve said: "I'll think about this a lot more when I'm next travelling along a motorway".

Thanks to Steve Kelly for doing the mathematics.

Kevin Williams
Survival Skills Rider Training

...because it's a jungle out there

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