Motorways: Step 3 - Defensive Driving

1. Advancing Your Driving Skills:
Introduction

2. Reading the road:

1. Introduction
2. Safe Speed
3. Safe Space
4. Location

5. Time
6. Other Road Users
7. All Weather
8. Try Not To Stop!

9. Limit Point
10. Cornering
11. Project

3. Eco Driving:
Overview

4. Sat Nav:
Sat Nav Use

5. Overtaking:
Safe overtaking

6. Dual-Carriageways:
2. Introduction
2. DC v Multi-Lane

7. Motorways:
1. Introduction
2. Smart Motorways
3. Defensive Driving

DRIVING HOME PAGE

Defensive driving on motorways

The video above gives some examples of taking a defensive approach to motorway driving.

Joining the motorway

There is sometimes controversy about whether or not drivers should make a right blind-area check when joining a motorway via a slip-road and acceleration lane. The information below taken from an article written by John Farlam in 2018 explores this issue to help you decide a safe course of action for yourself.

If you are not 100% certain about the position and movement of traffic in all three lanes as you join the motorway you face a significant risk, particularly if a vehicle moves across from lane-three or lane-two towards lane one. Reliance on mirrors alone will result in any vehicles moving across the motorway lanes being hidden in a blind-spot.

If you rely on mirrors alone your safety is totally dependent on other drivers seeing you, correctly anticipating your intentions and taking appropriate action. I don't know about you, but I've never been one for trusting my life to total strangers…

Without a blind-spot check to the right when approaching a motorway there is a significant risk from 'unseen' traffic.

But… On the other hand…

The argument against glancing back is that you will travel a considerable distance without watching the road ahead. This can be a significant risk because you will be 'driving blind' - at 70 mph a two second glance will cause you to take your eyes off the road ahead for about 65.6metres!

Figuring things out...

Let's think about the figures… (for ease, all figures are approximate to within 0.5 metres and assume good driving conditions and average reactions).

If it takes two seconds to complete your glance back and the vehicle in front starts to brake at the moment you look away from the road ahead (in order to glance back) it will have been braking for two seconds before you realise that it is slowing down. It’s estimated that from the point of braking it takes 4.7 seconds to stop from 70 mph (some vehicles may stop in a shorter time). This means that it will stop in 2.7 seconds from the point at which you recognise the emergency ahead.

Without a degree in physics I’m unable to calculate the distance that the vehicle ahead will cover in the final 2.7 seconds of braking but will guess at 25 metres. This means that the vehicle ahead will stop in around 20 metres from its location at the point when you see its brake lights.

At this point, because the vehicle ahead has already been slowing down under braking for two seconds, your two-second gap will now have reduced to one second or less (again a physics degree would be needed to calculate the exact distance/time). This means that the vehicle will now perhaps, only be 30 metres ahead of you. If we freeze time at that moment the vehicle ahead will stop about 50 metres from your current position (made up of the remaining 30 metres separation gap + the 20 metres that the vehicle will cover before it stops).

At this point you still have to think and brake – which will take you a 100 metres before you can stop… OOPS! If the vehicle in front stops in 50 metres from your current position and it will take you 100 metres to stop you are in big trouble!

Reference: for those with a grasp f physics/mathematics who might want to improve on my approximate calculations:

http://www.roadsafetyknowledgecentre.org.uk/help-forum/159.html
https://arachnoid.com/braking_physics/index.html

Crash!

If the vehicle ahead started an emergency stop at the exact moment you started to glance back and you are leaving a two-second gap the figures above suggest that you may only have a a few seconds left to live...

So I'm wrong in my suggestion that drivers should glance back and that glancing back is inherently dangerous at speed...

Well no, and I'll explain why.

Safety and risk

All driving involves risk we mitigate that risk by making choices.

Perhaps the first choice when joining a motorway is following distance. The two second rule is cited as the minimum distance that you should leave to allow a safety margin, but there are many factors that will determine your actual following distance,

For example, would you be happy leaving the absolute minimum gap if you were tired or unwell? If it was raining? If it was misty? If you are in a strange area and looking for signs?

Or perhaps, if you know that you are travelling at speed towards a known high risk situation?

Motorway driving is a bit like flying. With flying the greatest risk is when taking off and landing, on motorways (discounting road works) the greatest risk is when joining and leaving. Given that we know this it doesn't make sense to leave the minimum safety margins in these situations.

My first point then, is that your forward safety gap when entering a motorway will ideally be 3 or 4 seconds.

Incidentally, I sometimes encounter an argument to extending the forward gap... "OK John, but the car behind will start following too close".

The reality is that it's very rare for the car behind not to be too close, regardless of your forward gap; so all the more reason for extending the forward gap.

Normally arguments like this are a sign that someone is 'defending their position' rather than thinking the problem through.

A three or four second gap in itself mitigates all of the figure-work above and creates time and space for safe blind-area checks. But there is more to it than that.

The reality

The reality is that the vehicles ahead of are not likely to suddenly do an emergency stop without you being aware of a problem ahead of them, at least not if you're looking well ahead and driving defensively. Extending your safety gap to three or four seconds when joining the motorway creates more time and space, but you are not depending on this alone.

Active forward observation will often warn of problems ahead before the drivers in front of you have seen them. If you have experience of fleet or other full-licence-holder training you will be well aware that one of the main issues with many drivers is that they do not look, and plan, far enough ahead - especially on high-speed roads.

I don't think there are any statistics, but my experience would suggest that the most likely cause of an accident joining the motorway will be when drivers are not aware of what's happening around them, particularly the joining drivers being unaware of fast traffic on the main carriageway..

The video below shows a classic example, you might have to watch the clip a couple of times to spot the action as the clip is fairly low resolution.

A car joins from the left and then almost immediately moves out to pass a large vehicle. Unbeknown to the driver there is a vehicle in lane two hidden in the blind-spot…

Had this driver had one or two glances back in the slip road or even the acceleration lane they would have been more aware of the position and movement of traffic that was already on the motorway and the accident would have been unlikely to happen.

The glance back when joining a fast moving road via a slip road is a potential life-saver. It barely takes longer than a mirror check. Of course mirrors are equally important, but can't replace the shoulder check.

If the time taken to make a shoulder check leads to a critical situation it's not the shoulder check that's at fault – it's the fact that the driver is not leaving an adequate forward safety gap for the situation and speed or not 'reading the road' ahead. Removing the shoulder check won't fix these problems.

This is the end of Unit 5. Next, Unit 6 - Manoeuvres.