As can be seen by all the following comments, this is a
very complex subject to which there is no definitive answer. The
figures shown are a very, very rough calculation which tie in with those
featured in the current UK Highway Code. UPDATE (August
2019) – this information does seem to change
on a regular basis so, for the latest official data from UK
authorities check out the government site at –
https://www.gov.uk/guidance/the-highway-code/general-rules-techniques-and-advice-for-all-drivers-and-riders-103-to-158#rule126 Weather/road conditions, type and condition of vehicle,
load etc., not to mention the age, health and mental ability of the driver
can all affect the time taken to react and stop. If nothing else, these do highlight the amount of time and
distance required to bring a moving vehicle to a standstill. The first
comments, sent in by John Whittle, really do
emphasis this point. Regarding driving speeds and converting
miles per hour into feet per second. Take the speeds figure; for example 30 mph.
Halve this speed to give you 15. Add the two figures together and this is
approximately how far you will travel in one second at 30 mph
i.e. 45 feet. At 70 mph we do the same. 70 plus half of
70 which is 35, add the two figures together means that at 70 mph we will
travel 105 feet in one second. Makes you realise the reality and
relationship of speed, time and distance, and why so many people end up in
a pile up.
An even easier way to convert speeds into feet per
second has been sent in by Ken Jarrett. Halve
your speed and you have the answer in yards/sec. almost exactly. To
convert this into feet per second just multiply by 3. Example 1: 60
mph (halve speed = 30 mph) = 30 yards per second: multiply by 3 = 90 feet per
second.
Example 2: 70 mph (halve speed = 35 mph) = 35
yards per second: multiply by 3 = 105 feet per second.
The charts are featured at the foot of this
page but the comments, hints and tips prior to that make very interesting
reading!
Marin P writes – I am an Advanced Tactical Driving instructor and from my
own experience and experiments on and off the track, I found that on the
driving range, during the training sessions, and when the trainee driver
has all the senses “up and running” and is in the is in the “alert”
mode, the reaction time is somewhere between 0.67 and 0.90 However, when the driver is not on the “alert” mode, the reaction time
could be anywhere from 1.0 and 1.5 sec. To the stopping distance, a big contributive factor, after the Human
Perception Time and Reaction Time, there is a Vehicle Reaction Time (VRT)
and Vehicle Braking Capability (VBC) to be taken in consideration is the
road condition. The interaction between the VBC (tires) and road
condition is called static friction and in the stopping distance
equation is referred as Coefficient of Friction (Cf). A Cf between the dry asphalt and perfect tires is significantly
different than wet, icy, slick, sandy, dirty very smooth or oily
surfaces. Thus the ideal coefficient of friction (0.8) would be between a perfect
road surface and condition (that would be dry asphalt) with perfect set
of (new)tires, properly inflated. Although the same road conditions, the Fc decreases to 0.7/0.6/0.5 if
the condition of the tire is less then new. As for the stopping distance, in my opinion, the easiest way to
calculate is the following: Multiply the first digit of the current speed with itself. Such as: 20 mph – 40
feet
30 mph – 90 feet
50 mph – 250 feet
70 mph – 490 feet
90 mph – 810 feet
IMPORTANT
INFORMATION Although the information given below
was that shown in the UK Highway Code,I have received the following comments which should be
heeded. Your advice on
“thinking time” for drivers (shown below) is very much out of
date. Those of us who have the
misfortune of having to investigate and analyse road crashes have long
realised through numerous studies that an approximate thinking time for
driver’s is about 1.5 seconds
not the 0.67 seconds you are suggesting. The 1.5 seconds quote is an
estimation only as many factors do come into play as pointed out be
‘Joe’. Applying 1.5 seconds to your
20 mph (32 kmh) example will give a distance of 43.6 ft (13.3 m) not the
20 ft (6 m) quoted. Perception/reaction time is a very complicated
subject and should not be treated lightly. Hope this helps in your
understanding. John Ruller Warning
sent in by Joe – Just a note to the stopping distances of a
vehicle….these facts and figures are for a driver who is alert and well
rested, in good health and not impaired by alcohol or medication. Weather and road conditions, as well
as type of vehicle and weight of load being carried can also affect these
stopping distance.
I just want to point something out that seems to cause a lot
of confusion for many drivers – the difference between THINKING time
and REACTION time, and you might want to add this to your page. This bit of information from John Ruller is informative but
a bit confusing: Let’s try to clear this up. The 1.5 second figure that John
mentions is in fact a combination of both THINKING time and REACTION
time. If we look at the Highway Code, what we see labelled as
“thinking distance” isn’t really thinking time; for example: SPEED = 60mph “THINKING” DISTANCE = 60ft BRAKING DISTANCE = 180ft OVERALL STOPPING DISTANCE = 240ft
IF the driver is aware of the hazard ahead and knows
exactly what to do and when to do it, then The “thinking distance”
that the Highway Code talks about is actually the time taken to transfer
your foot (or on a motorcycle fingers and foot) to the brakes and begin to
apply them. So in reality it’s not “thinking distance”, it’s
REACTION distance – and it’s this reaction time that is commonly measured as
being approximately 0.7 second. However, to react promptly, the driver has to be already
aware of the hazard and ready to brake, just like when doing the emergency
stop for the examiner on test! You have to be thinking “I’m going to
need to brake” and just be waiting for the correct moment – in the case
of the driving test, your cue is the examiner banging his clipboard on the
dash! However, in real driving, the driver needs to SEE
the developing situation, IDENTIFY the
fact there is a threat and then DECIDE
if the required RESPONSE is to brake. This is the true “Thinking” distance and it has to
be added onto the reaction time and stopping distance to get the true (and
longer!) overall stopping distance that John mentions. If the driver isn’t aware of the hazard, either through lack
of concentration and tiredness, because they are being distracted by
something like a chattering passenger, radio, sat nav or mobile phone, or perhaps because the driver is
inexperienced and hasn’t been in that situation before, observations have
shown this thinking time can be up to two seconds! So if the driver’s not on the ball and it takes 2 seconds
for the driver to be aware of the problem, if the vehicle is moving at
60mph, the vehicle can travel 176ft (or 54m) before the driver even goes for
the brakes. So to calculate stopping distances from 60mph: Thinking Distance 176ft (54m) Reaction Distance 60ft (18m) (what the Highway Code
calls ‘thinking distance”) Stopping Distance 180ft (55m)
Overall Distance 416ft
(150m) including ACTUAL THINKING time & REACTION time! OK, that is pretty much worst case scenario for the inattentive
or inexperienced driver and a reasonably alert, reasonably experienced
driver will “think” a lot quicker and “react” sooner,
which is where that 1.5 second combined “thinking and reaction
time” figure John talks about comes from. But it’s sobering, isn’t it,
to see it will probably take even an alert driver longer to stop than the
Highway Code suggests? Just as well brakes have got better since those figures were
calculated! Kevin Williams,
Survival Skills Rider Training
TIP With thanks to Mr. Alex Beet Instead of having to learn all the data
contained in the table shown below there is a formula you can remember in order to
calculate the overall stopping distance, which is as follows:- x? ? 20 + x = Overall stopping distance in
feet. x = speed For example: If you are travelling
at 30 mph 30? ? 20 + 30 = (30 x 30) ? 20 + 30 = 900 ? 20 + 30 = 75 ft.
Simon Harding
has kindly sent in a easier way to remember these – Braking distances for cars. Why
have such a complicated equation to work out a very simple formulae?
i.e. 1 x 20 = 20ft 1.5 x 30 = 45ft 2 x 40 = 80ft 2.5 x 50 = 125ft and so on….
Overall stopping distances (that
is thinking + braking distance) 2 x 20 = 40ft 2.5 x 30 = 75ft 3 x 40 = 120ft 3.5 x 50 = 175ft and so on….
Somewhat easier to work out than square
roots! Remember –
Thinking distance is the same as the travelling speed in feet e.g. 30 mph = 30 ft. thinking
distance, 60 mph = 60ft thinking distance.
Peter Maddison has
sent in another suggestion along similar lines. I have an easier solution to Overall Stopping
Distance – 20 Mph X 2 = 40 Feet 30 Mph X 2.5 = 75 Feet 40 Mph X 3 = 120 Feet 50 Mph X 3.5 = 175 Feet 60 Mph X 4 = 240 Feet 70 Mph X 4.5 = 315 Feet Just Remember 2 / 2.5 / 3 / 3.5 / 4 /
4.5
Kevin Balding – I
notice that you have a number of ways of working out stopping distances
and all of them involve remembering numbers which frankly I’m not very
good at so here is a formula which will do it for you (Only one thing to
remember and I can do the maths in my head) 1. take the first number of the speed i.e. 5 for 50 2. divide by two and add one to the answer (This will give
you the 2, 2.5. 3 etc in one of the methods) 3. Multiply the answer by the speed = overall stopping
distance in feet 4 thinking distance in feet is the same as the speed 5. subtract speed from stopping distance to give breaking
distance all I have to remember is divide by two and add one. e.g. at a speed of 50 5/2 = 2.5+1 =3.5 x 50 = 175ft overall stopping distance take of the thinking distance (speed) 175 – 50 = 125
braking distance
Tip
When trying to visualise a
distance it is useful to remember that the length of an average car is
approximately 15ft, therefore, 75ft would be about 5 car lengths away.
Another way of judging distances has been sent in by Frederick Petersen – this uses a time factor.
“Pick a fixture on the side of the road (such as a bridge or
telephone box) and allow a gap of 2 seconds between you and the rear of the car in front.
This is attained by saying in your mind 1001, take a breath then 1002. Should
the weather be wet then it is advised that an extra second would make a major accident
less likely.”
Along similar lines Neil
Gebbie suggests –
“One thing I remember from a long
while back is for the 2 second gap, pick an object as the car in front
goes past and say, “only a fool, breaks the two second rule” it
takes 2 seconds to say and rings in your mind if you only say “only a
fool…….”
I must admit I have heard this before but
have found when trying to carry it out that cars keep popping in the gap in front and you
seem to be going backwards.
David
Redfern has kindly sent in the following comments including
suggestions on visualisation methods for calculating the distance required
which is probably a lot easier to remember when driving after passing your
driving test. I read your article on stopping distances and the laudable contributions by many experts, one or two I recognise having been in the driver training industry for a number of years. The calculations and debate are great, very informative however I refer to your DOS to Windows conversion and the principles of this web site, make it all understandable for novices. I have yet to meet anyone who has remembered the stopping distances they so studiously memorised when passing their test far less the Speed = Distance/Time calculation a week after passing their driving test. The fact is, it’s meaningless, just figures and as most of us learn by visualisation and experience it’s no wonder we forget it. I’ll also dispel one myth here, the ‘two second rule is nonsense’, although I haven’t worked it out, evidently its incorrect for all but one stopping distance in the Highway Code but what’s more important is that if drivers are continually spotting stationary objects to check their distance from the car in front, how much concentration are they devoting to actually driving? A cars stopping distance is the lowest common safety denominator. Ask any ‘advanced’ (I hate that elitist term) driver what the most important safety skill drivers should develop and they will almost all say “observations” (every single one I have asked) then ask them what it is in thick fog when observations are virtually useless and they will almost always say “stopping distance”, the last and default safety position. Unfortunately we confuse our learners by continually throwing figures at the problem when, by our own admission, there is still debate amongst the experts as to what should be considered the finite calculation. The vast majority of cars on the road are driven by reasonably healthy adults with good eyesight and reasonable reactions. The highway code advises us to beware of other vulnerable road users so elderly drivers, people on their mobile phones, with kids in the car etc. should all be given more time and space by us. For the rest of us, the visualisation of, say, 23 metres ought to be automatic, many guys will respond to “roughly a quarter of the length of a football/rugby pitch” (and yes I know they vary in length but it’s better than nothing). From that default the calculation becomes more manageable and I was taught that we should be looking at roughly 1 Yard for every mile per hour travelled. Times have changed and the Yard is now a metre, even more useful as at roughly 39 inches it’s about 7%(?) more which gives us a nice little advantage but although it becomes less accurate above 50mph or so its memorable and more important, recognisable especially when drivers can visualise 23M then mentally multiply up. One of you’re contributors mentioned the problem of people overtaking and pulling into the gap they had left. I would remind him/her that the distance you leave between yourself and the car in front is also the space you need to leave for overtaking vehicles. The reality is, no matter what gap you leave, if
someone is going to overtake you they will do it regardless of the gap in front of you, few people think that far ahead! SPEED | THINKING
DISTANCE See
comments above | BRAKING DISTANCE | OVERALL STOPPING DISTANCE | 20 mph | 20 ft. (6 m) | 20 ft. (6 m) | 40 ft. (12 m) | 30 mph | 30 ft. (9 m) | 45 ft. (14 m) | 75 ft. (23 m) | 40 mph | 40 ft. (12 m) | 80 ft. (24 m) | 120 ft. (36 m) | 50 mph | 50 ft. (15 m) | 125ft. (38 m) | 175 ft. (53 m) | 60 mph | 60 ft. (18 m) | 180 ft. (55 m) | 240 ft. (73 m) | 70 mph | 70 ft. (21 m) | 245 ft. (75 m) | 315 ft. (96 m) |
SPEED CONVERSION
MILES
PER HOUR | ?? | KILOMETRES PER HOUR
(APPROX) | | MILES
PER HOUR | ?? | KILOMETRES
PER HOUR (APPROX | 10 | = | 16 | 80 | = | 128 | 20 | = | 32 | 90 | = | 144 | 30 | = | 48 | 100 | = | 160 | 40 | = | 64 | 110 | = | 176 | 50 | = | 80 | 120 | = | 193 | 60 | = | 96 | 130 | = | 209 | 70 | = | 112 | 140 | = | 225 |
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