Posts Tagged ‘night’

Tractor / Trailer Turning Maneuvers and Turn Times in Night-Time Accidents

Wednesday, November 7th, 2007

Tractor trailers usually take three times as long or longer to accelerate as a regular passenger car. That coupled with the large bulky structure of a tractor trailer unit makes for a very long acceleration time to clear an intersection once the tractor trailer starts to pull out onto a roadway. The fact that the trailer wheels do not track directly behind the tractor wheels when a turn is being made means that a wider then normal turn has to be made in many circumstances. This can lead to the tractor going partially off of the roadway as it’s making the turn before straightening up in its intended lane. All of these factors need to be considered when trying to analyze just how the accident occurred and just what would be visible to the oncoming motorist during night time accidents. The following discusses the approximate turning times and tractor paths throughout the turn and the headlight visibility and orientation to the oncoming motorist through the turn.

 Studies show that average acceleration factor for a tractor trailer is approximately .05. That is an acceleration rate of 1.6 feet / second / second. This means that the first second the truck accelerates a distance of 0.8 feet, after 2 seconds it has accelerated a distance of 3.22 feet and after 3 seconds it has accelerated a distance of 7.25 feet. This shows that as times goes on the vehicle is accelerating to a higher speed and is gaining speed and covering a greater distance each second. So as seen in the table below if a truck accelerates for 10 seconds from the time that it starts until it reaches the point of impact it travels a total distance of 80.5 feet. The first 3 seconds it only travels 7.25 feet but the last 3 seconds it travels a distance of 41.06 feet.












Distance Total

From Start

1 =



2 =



3 =



4 =



5 =



6 =



7 =



8 =



9 =



10 =




 Depending on the roadway configuration and the amount of available sight distance the oncoming motorist may only see a portion of the truck’s total turn time prior to the impact occurring. What is important to know, in an accident where the oncoming vehicle runs into the side of a trailer, is where the oncoming motorist is located and where the tractor / trailer is positioned when the motorist could first see it. For instance if at a certain speed the motorist can see the tractor / trailers acceleration for the last 6 seconds from the time it travels from 12.88 feet to 80.5 feet, the motorist should be able to see that a tractor / trailer is entering the roadway and should start to slow down and be more attentive to the roadway. If, however, the sight distance limits the oncoming motorist’s view to only seeing the last 3 to 4 seconds of the tractor / trailer acceleration, the angle of the headlights might be such that the glare could veil the side of the trailer so that it might not be very conspicuous. It may appear to the oncoming motorist that there is just another vehicle approaching in the opposite direction not realizing the impending danger of a trailer angled across the their lane just beyond the headlights. This is where a thorough investigation determining the roadway geometry, the tractor / trailer acceleration characteristics, and the approaching motorist speed becomes necessary in properly analyzing this type of accident.

 A scene investigation using a total station or some other acceptable means of measurement to make a scale diagram is needed along with information concerning the acceleration characteristics and speeds of the involved vehicles. An acceleration test with the same or similar type of tractor / trailer and load that was involved in the accident can help determine the acceleration rate of the vehicle involved. A speed calculation can usually be done of the approaching vehicles speed by either crush damage analysis, speed from skidding or a combination of the two. Remember the important factor is what each of the motorist could see and at what point they could see it.


Todd Hutchison