Archive for the ‘Subjects’ Category

The Skid Mark / Crush Factor Method

Thursday, February 7th, 2008

When investigating an accident a common question that arises is, “How fast were the vehicles going?” If the skid distance of the striking vehicle and the maximum crush depth of the target vehicle are known, a simple estimate of the speed of the striking vehicle can be made. This is done by determining the impact speed of the striking vehicle by measuring the maximum crush depth of the target vehicle and inputting the distance in the Crush Factor Formula.

The Minimum Speed Formula uses the pre-impact skid distance of the striking vehicle and the roadway drag factor.

Both the Crush Factor Formula and the Minimum Speed Formula are then combined in the following way to determine the Striking Vehicle Start of Skid Speed.

30 - Mathematical Constant in the formula

d1 - Striking Vehicle’s Pre-Impact Skid Distance (measured in feet). Note: Measure the skid marks from the start to the point of impact (offset in the mark) and then subtract the wheelbase (front to rear axle distance of the skidding vehicle) from the skid distance.

f - The adjusted drag factor of the vehicle leaving the skid marks on the roadway surface. Note: if the vehicle is a passenger car, van, SUV or pickup truck and all four wheels left skid marks and the roadway was level, the roadway coefficient of friction is the vehicles drag factor. For a dry traveled asphalt surface the coefficient of friction is usually within the range .6 to .8 g’s. If the roadway surface is wet or has gravel on it, the coefficient of friction can be significantly less.

d2 - The target vehicle’s maximum crush depth (measured in feet from the normal undamaged position to the maximum permanent crushed position) of either the side or rear surface. Note: this calculation can not be used for head on collisions. This may only be used for t-bone or rear end collisions.

cf - The crush factor of the target vehicle is vehicle specific, but the average crush factor for the side and rear surface is 27. The crush factor values are based upon statistical analysis of 1000 vehicles involved in accidents where the speeds of the vehicles were verified by independent means.

The following example illustrates how the combined speed formula works:

 

The striking vehicle left 59.5 feet of pre-impact skid marks and had a wheel base distance of 9.5 feet. Subtracting the wheelbase distance from the total skid mark distance gives a pre-impact skid distance of 50 feet. The skidding occurred on a roadway that was level, dry asphalt. The drag factor was measured to be .7 g’s. The vehicle impacted into the side of another car and left 18 inches (1.5 feet) of permanent crush damage.

The combined formula was used to determine the start of skid speed.

This formula works as an approximation of the start of skid speed for situations where one car, van, SUV or pickup truck impacts into the side or rear of another car, van, SUV or pickup truck. This is a relatively easy way to determine if the vehicle was traveling in excess of the speed limit and to decide whether or not a more detailed accident reconstruction would be helpful. This simple estimate of the striking vehicle speed and the speed of the target vehicle can be confirmed by the conservation of linear momentum method. In collisions that involve vehicles that impacted either head-on or head-on at an angle, this skid mark / crush factor method can not be used. In those cases either conservation of linear momentum or some other method needs to be used to determine the speed of the vehicles.

Todd Hutchison

Geohazards: Rivers and Floods

Wednesday, November 7th, 2007

Hazard Characterization

Floods are high-water stages where water overflows its natural or artificial banks onto normally dry land, such as a river inundating its floodplain. Floods occur at more or less regular intervals in riverbeds and floodplains but also beyond them. Besides storm surges the two main types of floods are river flood and flash flood. Floods occur as natural phenomena when the volume of river runoff is so high, and the riverbed too small to contain the water masses. Floods, or high water stages, are most regular in springtime. Strong floods happen irregularly, in so-called re-occurrence intervals of 10, 50 or 100 years. But these intervals are only statistical averages. Heavy summer rainfalls can also lead to floods.

Floods have become an increasing problem for man’s environment since urbanization has altered natural drainage ways, straightened and even relocated river beds within their natural flood prone areas. Also impacting runoff rates of flood prone areas is the habit of increased soil sealing (asphalt parking areas, highways, commercial urban development, etc.) leading to a higher than normal flood hazard. This occurs as rainwater more quickly runs off directly into the streams and the water mass inflow via paved drainage ways and underground pipes to rivers, storm water runoff which is no longer delayed by natural soil retention induces “flash flood” like storm events.

Flash floods are the fastest-moving types of floods. A flash flood is a specific type of flood that appears and moves quickly across the land, with little warning. Heavy rainfall concentrated over an area, thunderstorms, hurricanes and/or tropical storms cause most flash flooding. Dam failures can also cause flash flood events. When a dam or levee breaks, a gigantic quantity of water is suddenly discharged downstream, developing strong destructive forces which may reach elevations previously undamaged by flood waters.

Flash floods can contribute to river floods, or can be caused by river floods, for example if an embankment collapses. Flash floods can happen anywhere but are mostly bound to river and stream drainage areas and are thus integrated into most government agency flood zone maps and the delineation of flood zones.

Risk Management

The most important part of flood risk identification and management is the flood-prone area delineation (extent). Flood-prone areas are those areas subject to inundation as a result of flooding with certain known frequency. The determination of a flood prone area requires considerable collation of historical data, accurate digital elevation data, and hydrologic discharge data calculated along a number of cross-sections located throughout a watershed. In addition to taking past flood events into account, it could be possible to derive river flood prone areas by area elevation modelling with the assistance of satellite imaging. Research institutions can develop a “flood prone area map” based on digital terrain models, river runoff, flood data and climate models. These are available to the public.

Flood Zone Determination

Flood Zone Determinations are usually provided by the state and local government engineering departments. Certificates of Elevation are provided by Professional Engineers, Professional Land Surveyors or Registered Architects from the private sector on a fee basis.

Flood Insurance

Flood insurance is recommended in flood prone areas. There are two types of coverage, structural and contents. Renters can buy contents coverage only. Flood insurance is available through the National Flood Insurance Program (NFIP) and information about this insurance is available through you local insurance agent. Don’t wait for the next flood there is a 30 day waiting period for coverage to take effect.

Dominick Amari , P.G.

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.

 

 

 

 

Sec.

 

 

 

Distance

Covered

 

Distance Total

From Start

1 =

0.805

0.805

2 =

2.41

3.22

3 =

4.03

7.25

4 =

5.63

12.88

5 =

7.25

20.13

6 =

8.85

28.89

7 =

10.46

39.44

8 =

12.08

51.52

9 =

13.68

65.2

10 =

15.13

80.5

       

 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 

Earthquake vs. Blasting

Thursday, February 17th, 2005

There are fundamental differences between earthquake activity and blasting activity. Blasting activity has a relatively small crater zone. The definition of crater zone is identified as the area where the hearth is actually physically displaced in a permanent fashion. The principle effect from blasting is the transmission of energy through the ground in the form of vibration. An earthquake has a more significant area where the ground is permanently displaced or shifted. The magnitude of an earthquake is considerably larger than that of conventional blasting. A different scale has been designed to measure the energy associated with an earthquake. This scale is called the Richter Scale (Modified Mercalli Scale). The Richter Scale is set up to define seismic events that increase with tremendous magnitude. Each number on the Richter Scale represents approximately 10 times more energy than the previous number. This relationship is set up to define a magnitude of earth motion event as it relates to its impact on a house.

Peak particle velocity is the normative measurements for blasting operations and is designed to measure the intensity of ground motion. On the peak particle velocity scale we slightly more than double the energy as we move from 1.0 inch per second to 2.0 inches per second peak particle velocity as contrasted with an increase 10 times when we move from 1.0 to 2.0 on the Richter Scale. The Richter Scale and Peak Particle Velocity are both curvilinear in nature; however, their curves characteristics are tremendously different. These differences are evidenced by the table below.

Richter Scale Magnitude Energy (Pounds of Explosives) Peak Particle Velocity
4 200,000,000 1457.47
3 20,000,000 230.99
2 2,000,000 36.61
1 200,000 5.80
0 20,000 0.91
-1 2,000 0.15
-2 200 0.02
-3 20 0.004
-4 2 0.0006

The physical soil displacement from an earthquake is tremendously different than the physical soil displacement from a blast. Earthquakes can generate significant physical displacement over a great distance while soil displacement are on the order of a few then thousandths of an inch when you are outside the crater zone of a blast. Based on this disparity of between blasting vibration and earthquake vibration it is easy to see that blasting requires specialized analysis, calculations and definitions, apart from the Richter Scale used to measure earthquakes effects on a structure. Analysis, calculations and definition associated with earthquakes do not directly transfer to blasting due to differences of frequency, intensity, and magnitude.

Peak particle vibration calculations prepared from Dupont formulas based on average distance for energy values that coincide with published earthquake data relating Richter Magnitude with pounds of explosive energy.

Wade Hutchison

Terrorism Impacts Blasting Industry Record Keeping

Tuesday, February 1st, 2005

During the last several years the events of September 11th have significantly impacted record keeping requirements of every blasting operation. ATF Regulations mandate a close tracking be maintained for all explosives.

The new ATF Regulations require Blast Logs, Travel Manifests, and Blasting Magazine Inventory Sheets to balance precisely. The additional record keeping requirements of the new laws can be laborsome. However, they are worthwhile. Contractors failing to comply with these changes have been penalized with fines and revocation of their licensure.

Accident Reconstruction: The Scene Investigation

Friday, August 17th, 2001

Accurate measurements mean accurate answers. If the at-scene investigation is done in a proper manner, it will assist the accident reconstructionist in determining the contributing factors to the accident. That is why the investigator who gets to the scene early on, while the physical evidence is fresh, is so important to the overall process of determining who is at fault. If the accident is serious enough it is best to have the accident reconstructionist do the at-scene investigation. If not, it is sometimes necessary to have adjusters or accident technicians to gather and record the physical evidence. In the event someone other than the accident reconstructionist gathers the scene evidence, that person may be needed to testify if the case goes to court. Since the majority of smaller dollar value cases do not go to court, the scene evidence gathering may be effectively done by someone other than the accident reconstructionist.

The first people on the scene include police officers, emergency technicians, special investigative units and in a perfect world, the accident reconstructionist. If the accident involved a fatality or had serious injuries the police investigators may be more thorough in their documentation of the scene data. This is usually the best opportunity to see the roadway markings while they are fresh. Some evidence such as debris patterns and anti-lock skid marks, etc. are short lived and is best seen at this time. During this part of the  investigation the traffic is stopped and the investigator has time to log more information.

The second type of investigator that arrives on the scene, including insurance adjusters, private investigators, and accident reconstructionists, can still gather much useful evidence. Even though some evidence might be gone, other evidence can be gathered and used together with the police measurements and at-scene photographs to determine what happened.

Types of evidence that can be gathered include the location of pre-impact skid marks, offset marks and gouge marks. If gathered properly, these can help the accident reconstructionist to determine the pre-impact direction and speeds of the vehicles, the types of evasive action used by the drivers and ultimately who was at fault in the accident.

The Scene Investigation

“Safety First” is the phrase that needs to occupy the thoughts of the investigator the entire time the investigation is being done. Even though this slows down the investigation, obviously it is a necessary component to being available for the next assignment. Though the scope of this paper is not to show
methods to follow when conducting a scene investigation in a safe manner, it is a reminder to use proper safety methods. Remember the investigator is out there to gather evidence and not to cause another accident. When arriving at the scene be sure to park completely off of the roadway and as far off the
shoulder as possible. Be visible with proper safety equipment, including safety vests and always be thinking “Safety First”.

Once at the scene and after remembering “Safety First”, size up the scene and decide what needs to be logged and preserved. The physical evidence can be extremely helpful in assisting the accident reconstructionist in his job. This evidence includes pre-impact skid marks, which will show the
direction of travel and assist in determining the speeds of the vehicles and offset skid marks and gouge marks, which help to show where the impact occurred. Other useful information includes scratch marks, oil or fluid spills, which can be helpful in determining the vehicles final rest positions. When using
conservation of linear momentum calculations to determine vehicle speeds, the impact and departure angles and distances are necessary. A good scene investigation will help with this. This field data will then be put on a scale diagram and through computer analysis, angles and distances can accurately be made.

In addition to taking good photographs that visually recording the physical evidence, there are two methods
most commonly used to log the data on a field sketch. These are the coordinate method and the
triangulation method. These methods utilize either electronic measuring equipment, steel tapes, or in some cases roll tapes.

The coordinate method uses one reference point, such as a utility pole or fire hydrant and a reference line such as an edge of pavement or painted line. It also uses the direction the measurement is taken from
using north, south, east or west. A field sketch is useful as a picture to show what items were logged and what measurements were made. Be sure and put on the sketch what reference point and reference line were used for the measurements and what direction is north. Once the reference point, reference line, and the north direction are established, and once the pertinent marks are put on the field sketch, the measurements can be made and logged on that sketch. These marks will be logged in relationship to the point on the reference line that is perpendicular to the reference point. Each point being located will have two
measurements. Each point needs to be measured to determine how far it is north, south, east or west from the zero point measured along the reference line and how far north, south, east or west it is away from the reference line. Record all measurements in feet and tenths of feet.

The other method used to log roadway markings is the triangulation method. This involves locating two
reference points and making measurements from each reference point to the point that is being located. In using this system, it is necessary to have two points that are separated and in an area that can best log all of the marks. These reference points need to be far enough away from each other longitudinally down the road and latitudinal across the road to give the best results. Since this type of method is done by using some form of electronic measurement device or two people, each holding one end of the tape, it is usually
more practical to use the coordinate method.

Todd Hutchison

Hail Information Now Readily Available

Wednesday, January 17th, 2001

New software now installed, gives VCE, Inc. the ability of accessing all 40 years of the hail records of the National Weather Service for any location in the United States. The records are current up to 3 months prior to the current date.

National Weather Service gathers hail information from every official reporting station. Reports from reporting stations include information from storm spotters located throughout the reporting area, law enforcement or emergency management personnel, media reports, and from the general public. Nashville, for example, has 11 weather stations, and there are about 400 spotters in Middle Tennessee. In many cities, most reports from a given reporting station are recorded as having occurred at the latitude and longitude of the reporting station. The accuracy of the data has not been independently confirmed.

To obtain information at a given location, one enters search information. The information entered, is the latitude and longitude of the location in question, the time period to be considered, the radius in miles of the search area, and the range of the size of hail. The smallest diameter of hailstone which may be used is 3/4 of an inch. The largest size is 5 inches in diameter. The program then issues a report showing the latitude and longitude, the date and time of day, and the size of hailstone for each occurrence of hail in the selected area.

An investigator may utilize the data obtained as dependable information for determining the probability of hail damage. The investigator must realize that a hailstorm is an event which occurs over an area while hail-fall reports are listed as point locations. Thus, a hailstorm could cover several square miles and deposit different sizes of hailstones, but there might be only one hail-fall report. Before the development of this software, the information was maintained at the National Weather Service, but was not readily available. It is true that not all occurrences of hail are reported. Nevertheless, the availability of reliable information about hail will greatly aid in the determination of actual hail damage.

Carl Hudson