International design rules allow car manufacturers a 10% error - both plus and minus - on vehicle speed as displayed to the driver.
If one further considers the needle, indicating speed is of such a width as to obscure 3 or 4 km/h or mph of the arc of speed values, plus one has high hopes the driver has eyes mostly for the road ahead, sparing but the briefest glance for the instrument cluster, then it is no wonder that none of us is ever doing precisely the same speed within a regulated zone. Digital speed displays use smoothing algorithms to ensure they are not jittering faster that Kylie Minogue on opening night. As a result they give the impression of accurate readings, when in fact they are hiding the wobble behind an LCD curtain.
As your tyres wear from brand new and unworn to faded and scuffed, eventually reaching the legal minimum tread depth, their diameter shrinks, meaning your distance rolled per wheel revolution reduces by round 3% over the life of the tyre. Or, framed another way, as your tyres get older they rotate more, but cover less distance for each rotation of the gearbox output shaft, meaning your actual speed diminishes compared to your displayed speed. Then factor in manufacturing tolerances in the sensors (usually on the gearbox output shaft, but it can vary), your wheel rims and the tyres. It is a miracle of modern manufacturing and engineering that each car brand ensures we remain within the allowed 10% margin!
A few years back, all in the name of safety, some folk here in Australia wanted us to be smacked with speeding fines and demerit points as soon as we were 1 kph (0.63 mph) over the limit! To my utter shock the WA Chief of Police came out and, based on his understanding of the issues I've just outlined above, told them all to stop being so bloody stupid! I was in shock for weeks at his rational, science based and reasoned proclamation!
Just because you can measure and control something, it does not mean you should...
Measurement impacts behaviour.
A 10% margin on speed readings in cars and laws calibrated to the same error allowances would be logical to implement if we believe speed limits are a practical weapon in the war on dangerous driving. However, as a dear friend used to say: It is never a question of how fast you are travelling, it is a vital question of how fast you stop.
Miss Physics applies a series of laws amply represented by Mr. Newton's Laws of Motion. Unless you are a sub-atomic particle or a black hole, Mr. Newton's maths will model you just fine. Airbags, crumple zones and the million other smart things quality cars do these days in the event of heavy impact all exist to increase the time available to slow down so that energy is dissipated over a longer time duration, resulting in less crushing blows for us humans. This application of the Laws of Physics ensures the car absorbs as much of the impact energy as possible, while transmitting as little as possible to us soft humans.
Margins for error, aspects of accurate measurement. Sensible, guided by science decisions. The things we need to know precisely such as how much energy can a human absorb before sustaining critical injury? Things where an approximation will serve. Are we travelling at 68.5 or at 70.9? Decisions and actions, shaped more by the consequences of what happens when things go wrong, rather than an arbitrary horror when an arbitrary definition of perfection is not attained for some reason for which Miss Physics cares not and Mr. Newton has maths not.
Anyone who has flown in a commercial jet has travelled on the ground at around 300 kph (190 mph) during take-off before proceeding to sip water from a plastic bottle (economy) or a delightful selection of French reds (business class) while blasting through the air 38,000 feet above the earth at an air speed around 800 kph, being around 500 mph. Instant death!? Horror heaped on horror? Life in the mouth of the volcano? Unless one is travelling with one of the planet's more questionable airlines, no. You will arrive as intended at around the time intended, and proceed with your day roughly as intended. All a bit approximate? Sure. Yet we humans can live with a sensible amount of ambiguity, so that as a race we do not enter a group mind-state of crushing OCD rendering our very humanity a stain upon the universe.
Politicians and the FIA appear to want to live within this universe of perfect definitives, except when they wish to allow themselves an error margin which would allow the Titanic to be in the South Pacific that night rather than the North Atlantic. Utterly ruthless on everything but themselves...! So to the blazers of the FIA one can only but ponder.
There is a glorious 1970's picture of Ronnie Peterson's wife, Barbro, monitoring practice lap times with a clipboard, pencil and mechanical stop-start push button stop watch. A leading driver in a top team and his wife was pressing buttons on a mechanical stop watch.
Anyone wish to hazard a guess as to the operator induced timing error each lap with that approach? A second? Half a second? A quarter? Assuming she was focussed, was at a perfect, unchanging angle to the finish line, to eliminate parallax error, and did not suffer any neurological issues (increasing her reaction time) the very best she could have achieved would see a variance of around plus or minus two tenths of a second per lap. Something around half a second is more probable. Tyres going off? Missed gear coming out of the final corner, changing fuel load, tail wind on the back straight? Or simply the limits of a mechanical stop watch and an attentive wife? Did that margin for error destroy racing back in those raw days?
Ronnie (pictured driving the Tyrrell P34 at Monza in 1977) was a remarkable driver. He and Mario Andretti were dominating the 1978 season in the glorious ground effect Lotus 79. At Monza, Mario took pole while numerous mechanical issues saw Ronnie forced to resort to the older 78, which he managed to qualify 5th.
24 cars qualified for Monza that year. As the formation lap was nearing completion the flag - no lights back then - was dropped before the last had rolled into their grid slots, causing a chaotic start. Approaching the first of the 70's added chicanes, at that time named the Variante Goodyear now called Variante del Rettifilo, Riccardo Patrese, in his Arrows, touched James Hunt's McLaren causing it to spin into Peterson's Lotus, launching him into the barriers on the right hand side of the track. As the multi-car crash unfolded the cars from the rear of the grid were closing at high speed. Vittorio Brambilla, who had started his Surtees from the back of the grid, arrived fast and was instantly, desperately trying to avoid the cars spinning in front of him. Running out of space he speared into Ronnie's stranded Lotus causing it to promptly burst into flames.
Hunt leapt from his stricken McLaren heroically pulling Ronnie from the burning wreck. Ten cars were now littered across the track, with the Lotus furiously burning.
In panic the Italian Police formed a human wall across the track, stopping everyone, including surgical advisor Sid Watkins, from approaching the stricken drivers. Eventually after a delay nearing fifteen minutes an ambulance arrived to take Ronnie to hospital.
With his legs a mess Ronnie underwent extended emergency surgery, before being placed in ICU in what was believed to be a stable condition. At around 4:00am Watkins received a telephone call saying Ronnie's condition was deteriorating. Before Sid could do anything useful Ronnie was clinically brain dead. Later it was discovered a fat embolism in his thigh resulting from the massive crash injuries combined with the extended surgery plus, most damningly, the delay to starting treatment, were the factors combining as the probable cause of death.
Wrong start procedure, delayed responses, no fire marshals (it was Hunt who performed the rescue), police blocking aid, not helping it access the scene. No ambulance for fifteen minutes... So many errors. In combination, a disaster of a start, ten cars seriously damaged, several drivers injured and a great talent in Ronnie Peterson dead. A margin for error that was a bloody rent in reality into which the FIA poured every possible error during mere seconds of tragic events.
The margins for error here were so massive one could have flown a dam-busting Lancaster Bomber through them, and not lost any paint from the fuselage. At the time it was yet another horror incident serving to accelerate many changes. The move of Sid Watkins to a full time senior medical position, the medical safety car following the field at starts, trained fire marshals and many more alterations over the following years. Each safety revision aimed at eliminating more of the margin for error, and thus making safety inbuilt, not an unplanned accidental benefit.
Some things demand to be measured and as far as is reasonably possible controlled. Other things need to be monitored for signs of concern, but not overly fussed about. Yet others can be measured for all the wrong reasons, thus producing results unexpected, and occasionally unforeseen disastrous outcomes.
On the public road we are afforded a reasoned 10% margin for error with our actual speed. This is then measured against an arbitrary speed zone limit set by some jobs-worth buried deep within an indifferent government department. Usually with no recognition of Miss Physics, safety-leading road design, quality of driver training, condition of the vehicle in question, or current local weather conditions.
Most higher education establishments consider a mark above 75% as either an "A" grade, or a "High Distinction" depending on local messaging beliefs. By contrast, would you be likely to board a plane where the airline assured you that "nearly 75% of our flights make it to a safe landing"...?