Thursday, April 5, 2012

Safe Following Distance On the Highway


When driving on the highway, we need to keep a safe following distance between our vehicle and the vehicle in front of us. Because speed is distance divided by time, then 1 second at high speed can be can be very decisive.

Some consideration in determining the distance to the vehicle in front are:
• Driver reaction time
• Vehicle braking distance

Two-second rule is ussually used as guidence for following distance on the highway. I often use the two-second rule for drafting behind big rig to save fuel.

The following chart describes the distance of 2 seconds at any given speed:

Vehicle Speed
Distance in 2 Seconds
kmh
mph
meters
feet
50
31
28
91
60
37
33
109
70
43
39
128
80
50
44
146
90
56
50
164
100
62
56
182
110
68
61
200
120
75
67
219
130
81
72
237
140
87
78
255
150
93
83
273
160
99
89
292
170
106
94
310
180
112
100
328

From the above chart, it may be determined roughly that for the speed in kilometers per hour (kmh), the safe distance in meter is speed divided by 2. For example:

For a speed of 60 kmh, a safe distance = 60 / 2 = 30 meters, 33 meters in the chart.

For a speed of 100 kmh, a safe distance = 100 / 2 = 50 meters, 56 meters in the chart.

For speed in units of mile per hour (mph) then the distance with the vehicle in front is roughly about 3 times the speed, the distance in units of feet. For example:

For speed of 37 mph, a safe distance = 37 x 3 = 112 feet, 109 feet in the chart.

For speed of 62 mph, a safe distance = 62 x 3 = 186 feet, 182 feet in the chart.

As we can see form the chart, it appears also that the speed in mph is about the same as distance in meters. So that it can simply be estimated, for example:

For speed of 50 mph, the safe distance is about 50 meters, 44 meters in the chart.
For speed of 99 mph, the safe distance is about 99 meters, 89 meters in the chart.

But how to measure the distance while driving? If the highway has distance markers with range of 20 or 50 meters (60 or 150 feet), it can be used to judge the distance between vehicles.

Another way is to judge distance by an object at roadside. Look at the object, eg tree, it was passed by the vehicle in front. See the picture below, the yellow car passes a tree by the roadside. Your car is a red car behind the yellow car.


Then count by saying ‘one thousand one’ and ‘one thousand two’ in your mind. When you are completed saying, and the tree is about at your car side, then your distance to the vehicle in front is in conformity with two-second rule, as shown below.


When condition is no ideal due to rain, mud, snow, etc., then multiple the range 3 times of 2 seconds or equal to 6 seconds. Poor visibility and slippery road will make it harder for driver to react and stop the vehicle.

Many rear end collisions are caused by vehicle in the back following too closely. In some countries, if you follow too closely (tailgating) you may become subject to legal sanctions. Some transportation organizations even suggest to use three-second rule for better safety.

If there is a vehicle too closely behind you, move to the other lane to allow that tailgating vehicle to pass. It is much better than to bear the risk caused by the carelessness of other driver.

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