Jeff Steiner's Americans in France.
Resource for people that would like to live or travel in France.

Americans in France

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Narrative

Distance Calculations

In general, you adjust your speed according to the situation. Especially in cases of danger or reduced visbility, you must slow down. This means as you crest a hill, turn a sharp curve, go down a steep slope, or hit a rough patch in the road, you must slow down.

Reaction Time

Driving involves 3 elements: the vehicle, the environment (road, conditions, other drivers, ...), and the driver. When something happens, you need some time to react to it and decide what to do before anything actually happens. In general, reaction time under normal conditions is about 1 second. The consumption of alcohol, drugs, and certain medications can increase the time required to react. Don't be fooled on the driving test when they ask you if bad weather increases the reaction time. No -- the reaction time is the same, but the braking time will be greater.

As a general rule, a typical driver takes 1 sec to react. The distance travelled in that 1 sec can be estimated by a simple equation: Take the ten's place of the speed and multiple by 3. For example, at 90 km/hr, you travel 9 x 3 = 27 m per second.


Minimum Stopping Distance

The distance it takes to stop a vehicle (distance de freinage) is proportional to its speed, its weight, and the road conditions. ABS (Anti-lock Braking System, and called ABS even in French) does not reduce the braking distance; it just prevents skidding so you keep better control while braking. A vehicle that is heavily loaded will have a larger braking distance than an empty one. A wet road cuts the traction of the tires in half, effectively doubling the braking distance.



On the test, you may be asked to calculate the minimum stopping distance at a given speed. For that, you will need to memorize the following equation: Take the ten's place of the speed and square it (dry road). If the road is slippery, add another 50% to account for the extra braking distance needed.

at 70 km/h...  7 x 7 = 49 m on dry road
at 70 km/h...  7 x 7 x 1.5 = 74 m on wet road

This is an approximate distance under normal conditions (alert driver taking 1 sec to react, no extra loading of the vehicle), to be used as a rule of thumb.


A wet road doubles the braking distance, but causes the stopping distance to grow by only 50%. Algebra lovers will notice that this isn't mathematically correct. The difference is because (a) the distance travelled during the 1 sec reaction time is a significant fraction of the braking distance but doesn't change with a wet road, and (b) these equations are rough estimates and are necessarily a bit crude.
  
MIN STOPPING DISTANCE
SPEED DRY
ROAD
WET
ROAD
30 km/h 9 m 14 m
50 km/h 25 m 38 m
70 km/h 49 m 74 m
90 km/h 81 m 122 m
110 km/h 121 m 182 m
130 km/h 169 m 251 m

Minimum Safe Distance

If the vehicle in front of you suddenly brakes, you need time to react. As with the discussion of braking distance, this is a function of your speed, the road conditons, etc. The official rule is to give yourself 1 second to react, then add another second for safety margin.

On the test, you may be asked to calculate the minimum safe distance between vehicles (called "intervalle de securité" in French) at a given speed. For that, you will need to memorize another equation: Take the ten's place of the speed and multiple by 6. Notice that this is the same equation as above for reaction time multipled by 2.

at 90 km/h...  9 x 6 = 54 m

This is an approximate distance under normal conditions (alert driver taking 1 sec to react, no extra loading of the vehicle), to be used as a rule of thumb.


There are two other tricks for knowing if the distance between you and the car in front of you is sufficient:
-- As the back of the car in front of you passes a fixed object like a lamp post or a sign, count the number of seconds before the front of your car crosses the same object. If you can count 2 seconds, the space between you is enough. FYI, while in English you count "one one thousand" or "one Mississippi" for each second, in French you can say "un Y" (remember 'Y' is pronounced 'eegreck').
-- If you're on the autoroute, the dashed lines on the outside of the road can be used to help estimate a safe distance. Two lines and a space between them is roughly the minimum safe distance for two cars travelling at 130 km/hr. Thus, if you can see two lines between you and the car in front of you, you are following at a safe distance.
  
SPEED MIN
SAFE
DISTANCE
30 km/h 18 m
50 km/h 30 m
70 km/h 42 m
90 km/h 54 m
110 km/h 66 m
130 km/h 78 m