There is no such thing as absolutely safe speed. Nope. None. Nada. Even if your speed is zero, you can still be stricken by meteorite, or a mine can burst under your car, or earthquake can strike, or tsunami can come pick your choice, or add your own one, the result is the same, regardless. There is no such thing as a safe life for that matter - lifes death rate is 100%! With this in mind, we can try to be more realistic with safe speed definition.
I would call the speed safe, if I estimate the chances of bad things happening on that speed being equal to such chances for zero speed (see above paragraph) or slightly higher. How much higher depends on personal risk aversion. How good is the estimate depends on personal driving knowledge and experience. That means objective safe speed does not exist. Safe speed is a highly subjective measure. What I deem safe can be considered unsafe by you, and vice versa. Worse, our opinions change as we gain more knowledge and experience.
What to do about that? Accept the fact that driving (and life!) is dangerous by nature and live with that. You can and should try to minimize your driving risk, but you cannot eliminate driving risk altogether. And the question of safe speed becomes the subject to risk/reward analysis, as many other things in our life.
Now, how one can arrive at the number? What are variables that define a safe speed? How fast is too fast, and how fast is still safe?
Official answer is simple anything above speed limit is too fast, no matter what. Period, no questions asked. As soon as it can be proved your speed is above speed limit, you are wrong and guilty, and you get pulled over. As easy as that. Authorities claim they have more versatile answer, when they introduce clauses about adjusting your speed with the road conditions change, or about not going too slow in the left highway lane.
But did you ever see a cop pulling someone over for going just below speed limit on an icy road? Or a cop pulling over a retired teacher, who is driving 10 mph below limit in the left lane under ideal environment? I did not. And I do not believe in versatility of official answer, until I do see those kinds of things on the regular basis. About as frequent as regular speeding tickets.
Until then, we are on our own to try and figure out what is the maximum possible speed that is actually safe under given circumstances? And, given this maximum possible speed, how fast we actually want to drive right now?
By and large, there are three major variables to this safe speed equation the driver, the car, and the road. BTW, this is why my forums are categorized this way. Let me talk about those variables in some detail.
The driver means everything related to driver his knowledge, skills, experience, physical and mental shape, mood, focus on the task at hand, his judgment ability, his personality, etc.
The car means everything related to car both in terms of design and shape, which allows or does not allow this particular car to drive at this particular speed. And this does not mean that car just can reach this speed, it means the car can maintain this speed with reasonable three-dimensional stability under given environment. It is not about engine at all (we have a joke in Russia: even a wardrobe can reach 200km/h if you let it slide down a big mountain), it is all about tires, suspension, steering, body stiffness - active safety in other words.
And the road means just that road conditions, environment, everything outside the car that affects or might affect its movement and, therefore, influences our decision about safe speed. Driving on a fenced highway obviously dictates speed different from driving inside residential block, driving on an icy road during midnight snowstorm different from driving on this same road on a warm, nice, and clear autumn noon, etc.
Our job as a driver is to constantly assess all those variables and decide what is the maximum safe speed for every one of them, and then to pick the lowest this will give you the speed that is safe from all three points of view. In other words, the maximum speed a particular driver on particular moment in particular car on particular road part can safely drive. A professional racer on a new performance car on a nice warm day would drive about the speed of a pedestrian on a tight residential lane, because he just has almost no room to react if children suddenly run into the road in front of his car.
All this does not mean you have to drive exactly at the maximum safe speed. Most of the times an experienced driver will drive slower than this, in order to not to push himself, or his car, or environment to the limit, and to give himself some room for error. It usually depends on the current trip goal. If you were sightseeing, you probably would drive much slower, and if you were delivering your wife to the hospital, you probably would push it to the limit or even further, which is a mistake, by the way.
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