## 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 wh
ile 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 mindWhen 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.