the anatomy and analysis of a typical pedestrian / bicycle ... · the anatomy and analysis of a...

Proceedings of the 2011 NATARI Annual Combined Conference, Harrisburg, Pennsylvania, October 5-7, 2011

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The Anatomy and Analysis of a Typical Pedestrian / Bicycle Crash Event

Mike W. Reade, CD Forensic Reconstruction Specialists Inc.

Institute of Police Technology and Management – University of North Florida Adjunct Instructor

Abstract A pedestrian crash investigation can be one of the most challenging and yet rewarding investigations to undertake. As with any investigation, trying to figure out where the area of impact is located can be very difficult and at times impossible to establish. During the collision sequence, a pedestrian will undergo a very dramatic and sudden change in velocity in a very short time. As the colliding vehicle and pedestrian interact, the pedestrian’s upper body will either, wrap around the vehicle’s front profile, or the pedestrian’s body will be projected forward and along a level trajectory, or takeoff.

After impact, a brief carry distance, the pedestrian released from the vehicle. Once the airborne phase is completed, the pedestrian will touch down on the road surface and commence to slide, roll or tumble to its final rest position. During this phase, the pedestrian experiences a significant ground impact that results in a sudden speed loss. This paper will break down a typical pedestrian crash event into meaningful collision phases and look at other factors that may influence your overall reconstruction analysis.

INTRODUCTION A typical crash involves a pedestrian who is either, walking, running or riding a bicycle when the impact occurs. In bicycle collisions, the bicyclist’s center of mass can be below or above the vehicle’s front leading edge (hood height) when impact occurs.

Although collision vehicles can be braking before, at/after impact, or not at all, the crash test data [1] researched as part of this paper, shows there are no significant differences in the final calculations regardless of driver action. When a braking vehicle strikes a pedestrian, there can be some vehicle/suspension loading which may show a slight widening of the vehicle’s tiremark(s). Should this occur, it will be helpful to investigators when locating where the pedestrian and the vehicle was positioned at impact.

The nature of crash scene evidence is delicate and it can be destroyed by passing vehicles, the first responders, or any witness willing to assist. To preserve the trace evidence, investigators


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would be wise to limit access to the roadway as soon after the event as possible. All too often, this evidence can play a vital role later in the final analysis.

A typical pedestrian crash event will result in the pedestrian’s body conforming or wrapping to the vehicle’s front profile during the impact phase and before the airborne phase starts. This type of collision is referred to as a Wrap Trajectory [2, 3]. Here, the vertical height to the pedestrian’s center of mass must be located above the front leading edge (hood height) of the striking vehicle.

The crash test data suggests the pedestrian’s projectile velocity is less than 100% of the striking vehicle at impact. Figure 1 shows an example of a wrap test trajectory.

Figure 1 - Wrap Trajectory Test – Albuquerque, NM, August 2009

The other pedestrian trajectory occurs when the pedestrian’s center of mass is below the vehicle’s front leading edge (hood height). This collision type is referred to as a Forward Projection Trajectory [2, 3]. With the pedestrian’s center of mass below the vehicle’s hood height, one would expect the pedestrian to acquire nearly 100% of the vehicle’s striking speed.

Figure 2 shows an example of a forward projection trajectory test. Clearly, when the pedestrian’s center of mass below the front leading edge, the pedestrian can be accelerated to the vehicle’s striking speed.

Investigators should be aware the pedestrian’s body may become trapped on the vehicle at any time during the impact and carry phase. To address these concerns, it is necessary to conduct a full crash scene documentation of all trace evidence.


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Figure 2 - Forward Projection Trajectory Test - Wisconsin, August 2009

After reviewing several crash test videos, it is clear that pedestrians do not immediately become airborne upon impact. Rather, during the wrap or the forward projection phase, the pedestrian is carried a short distance before the airborne phase commences.

As part of each IPTM (Institute of Police Technology and Management) Pedestrian/Bicycle Crash Investigation Course, the test data which is collected from each course is helpful when analyzing when analyzing this type of collision.

ANALYSIS METHODS After investigators have located the area of impact, they measure the total distance from first impact to the pedestrian’s final rest position, or the “Total Throw Distance.” This distance includes the Impact, Carry, Airborne, Ground Impact and Pedestrian Sliding Phases.

For this research, the many calculations were performed using the PEDBIKE 2000 Plus® - Pedestrian/Bicycle Specialty software [4].


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This paper will focus on the work of Dr. John Searle [5, 6, 7] as expressed in his research. The Searle Minimum Formula can be adjusted to address each topic as required.

(Refer to Appendix A for an explanation of variables used in each formula.)

μ

=+ μmin 22 gsV1

(1)

PEDESTRIAN CRASH PHASES & ANALYSIS

Pre-Crash Phase The pre-crash phase involves the movement of both the vehicle and the pedestrian. It is important to note that regardless what each party is doing before impact, the same amount of time is used for each unit to reach impact during any time and distance calculations.

Should any of the participants brake before impact then investigators will need to consider the amount of pre-impact speed loss in their final speed results. Once the vehicle’s speed at the start of braking is determined, a further time and distance analysis will allow investigators to place each participant at a location where the driver’s perception and reaction is likely to have occurred.

Pre-Crash Analysis During the pre-crash phase, the investigator must account for any speed loss by using the slide to stop formula.

V 2fgd= (2)

Where the results “V” are expressed in “feet per second” and “f” is the roadway friction value, “g” is the gravitational acceleration and “d” is the distance in feet that the vehicle skids on the road surface. Once the pre-crash speed loss is determined, investigators can combine these results with any post-impact speed calculations using a combined speed formula.

= +2 2C 1 2V V V (3)

Where the results “VC” are expressed in “feet per second,” and “V1” represents the pre-impact speed loss in feet per second while “V2” represents the pedestrian’s throw distance results in feet per second following the collision.

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Airborne Analysis

Unlike a true vault situation, it is not critical to know where the pedestrian first touches down after the airborne phase is completed. Since pedestrian airborne formulas include the impact, carry distance, airborne and the pedestrian sliding phases, investigators need not worry themselves, by trying to locate the location for first touchdown on the road surface. Rather, it is more important to locate where the pedestrian was when impact occurs so the total throw distance can be determined.

The crash test video [1] collected during the crash testing is reviewed in the cSwing [11] computer software. This software program is designed to analyze golf swings. However, because of the software’s unique ability to scale distances and measure angles directly on video footage, this software became a useful tool to gather pedestrian carry distance and pedestrian takeoff angles directly from the crash test video.

Figure 7 is an example of the cSwing program’s main screen with a video example.

Figure 6 - cSwing Program Screen Capture Showing Measurements

Much discussion has been made about what the pedestrian takeoff angles are or should be as the pedestrian becomes airborne. Although, investigators have been interested in takeoff angles for many years, it is becoming more apparent the pedestrian’s takeoff angle is not required using the method in this research to estimate the pedestrian’s projectile speed.

To determine the pedestrian’s projectile speed, the Searle Minimum Speed Formula is used. The total throw distance from impact to final rest along with the pedestrian’s sliding friction value


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are required to complete this calculation. The following formula can then be expanded to include the overall effects of the roadway slope, the change in vertical height between takeoff and touchdown, the pedestrian carry distance and the vehicle’s and pedestrian’s weight.

min 22 gsV1μ

=+ μ

(5)

The projectile velocity results “Vmin” obtained using this formula are in “feet per second.” Where: “μ” is the pedestrian sliding coefficient of friction, “g” is the gravitational acceleration and “s” is the total throw distance from impact to the pedestrian’s final rest.

Once investigators have determined the velocity required to project the pedestrian a total throw distance, compare these results to the results obtained using formula 8, or the pedestrian’s speed from sliding along the road surface.

Ground Impact Phase

As the ground impact phase commences, the pedestrian experiences a significant impact where the pedestrian loses some horizontal velocity [9]. Once this initial contact takes place the body slides, tumbles or rolls to its final rest position. Searle [7] has suggested similar observations during his research.

For this paper, the crash test data indicates the horizontal velocity lost upon initial ground impact is 6.6 mph (10.6 km/h).

Ground Impact Analysis

As a follow up to research already conducted by Searle that discusses the horizontal speed loss on impact with the road surface, the following formulae are used to address these issues. An excellent reference source on “Airborne Analysis” is Fundamentals of Traffic Crash Reconstruction, Volume 2 of the Traffic Crash Reconstruction Series, and authored by Daily, Shigemura, Daily [12].

ov v sin= × θ (6)

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Pedestrian Sliding Analysis

Determining the pedestrian’s speed loss while sliding along the ground requires one to know the sliding distance while travelling along the road surface. Also needed is an appropriate pedestrian friction value between the sliding pedestrian and the road surface on which it slides.

V 2 gd= μ (8)

The pedestrian’s velocity from sliding results “V” obtained using this formula are in “feet per second.” Where: “µ” is the pedestrian’s sliding coefficient of friction value, “g” the gravitational acceleration and “d” is the pedestrian’s total sliding distance in feet along the road surface.

After determining the pedestrian’s speed loss while sliding along the ground, we can compare this to the vehicle’s impact speed. In all crash tests reviewed, the speed determined for the pedestrian’s sliding using this method is less than the impact speed of the test vehicle as recorded by the Vericom Performance Computer [13], the Police Radar unit, or the total vehicle braking distance. The loss of horizontal speed upon initial touchdown following the airborne phase explains why this occurs.

Interestingly, when comparing the pedestrian’s sliding velocity to the Searle Minimum Formula results, the results of formula 8 are less than the Searle Minimum Formula results. This is also true when comparing these results to the striking vehicle’s speed.

Pedestrian Friction Values

Pedestrian friction values originate from a variety of sources. One of the most recent discussions by Searle [7] outlines the use of a sandbag method covered with different clothes travelling upon different road surfaces.

Table 1: "Sandbag" Coefficient of Friction on Different Surfaces [7]

Surface Dry Wet Frost

Coarse Asphalt 0.78 0.78 0.58

0.73 0.67 0.30

Fine Asphalt (New) 0.66 0.67 0.12

Fine Asphalt (Worn) 0.70 0.72 0.18

0.67

Concrete 0.77 0.64

Anti-Skid 0.94 0.90


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(Ped Crossing)

Grass 0.60 0.55

0.47

0.40

Table 2: Hill's Frictions Results (Adjusted) Fine Textured Asphalt, Dry [7, 14]

No of Tests Average Friction

Serge Jacket & Trousers 19 0.702

Body Warmer, Jumper, Trousers 21 0.723

Nylon Jacket & Trousers 10 0.587

Woollen Boiler Suit 10 0.750

Rubberized Cotton Jacket, Wool Trousers 12 0.735

Table 3: Bovington Friction Results (Adjusted) [7, 15]

Airfield Fine Textured Asphalt – Damp Conditions, One Test

Nylon Rain Suit 0.532

Leather M/C Suit 0.562

Nylon M/C Suit 0.608

Woollen Boiler Suit 0.633

Rubberized Cotton Jacket, Wool Trousers 0.612


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Table 4: Pedestrian Friction Values from Personal Testing [1]

Crash Test Dummy Dressed in Cotton & Jean Materials

Ped/Bike Class Albuquerque, NM – Average of 10 Pulls 0.67

Ped/Bike Class Fort McCoy, WI – Average of 6 Pulls 0.59

Ped/Bike Class Augusta, ME – Average of 10 Pulls 0.50

Ped/Bike Class Sewell, NJ – Average of 10 Pulls 0.54

Ped/Bike Class Scotch Plains, NJ – Average of 10 Pulls 0.59

Ped/Bike Class Narragansett, RI – Average over Several Surfaces 0.66

Table 5: Pedestrian Friction Values from CATAIR Testing [16]

Crash Test Dummy Dressed in Nylon, Cotton & Jean Materials

CATAIR Winter Testing, Riverview, New Brunswick (Wet Asphalt) 0.580

CATAIR Winter Testing, Riverview, New Brunswick (Snow/Slush Mixture) 0.526

CATAIR Winter Testing, Riverview, New Brunswick (Packed Snow) 0.449

SPECIAL CONSIDERATIONS

Vertical Change in Height between Takeoff and Touchdown

Normally a change in the pedestrian’s vertical height of center of mass between takeoff and first touchdown will occur through some three or four feet. Any differences in speed because of this short distance are very small and insignificant. However, when there is a greater difference in the vertical height difference, there will also be a greater difference in the horizontal speed loss at first touchdown as the pedestrian commences its sliding distance. Understandably, the greater the vertical height difference, the shorter the pedestrian’s forward movement, or sliding distance there will be. One can also conclude that the touchdown angle between the pedestrian and the roadway will increase as the vertical height difference increases.


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Figure 8: Vertical Height Difference Graphic from Visual Statement EdgeFX Software

The following formula will consider the effects of vertical change in height between takeoff and touchdown [6, 7].

min 22 g ︵s H ︶V

1μ −μ

=+ μ

(9)

Where the results “Vmin” are expressed in “feet per second” and “µ” is the pedestrian’s friction value, “g” is the gravitational acceleration, “s” is the pedestrian’s total throw distance in feet and “H” is the vertical change in the pedestrian’s center of mass between takeoff and touchdown.

Level Surface

Lower Surface


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Speed Analysis While Considering All Effects

Investigators will face many challenges during the course of their investigations. Situations will arise when you will ask yourself “Do I consider the effects of carry distance, roadway slope, or vertical height differences?” at some point during your analysis. More importantly, do these considerations effect the overall analysis and if so by how much.

To answer these questions, one might consider the approach that Dr. Searle’s research [7], where he provides solutions to each separate issue. So, if we expand on Searle’s work, we arrive at one formula that will address all these situations.

( ) ( )( )

min 2

1g C D2 sM

HV

1

sm

M

o Sin⎛ ⎞⎛ ⎞⎛ ⎞

μ −⎜ ⎟⎜ ⎟α ± × α⎜ ⎟⎜ ⎟μ⎝ ⎠⎝ ⎠⎝− μ

⎠+=

+ μ (10)

This one formula will consider the effects of vehicle (M) and pedestrian (m) weight (Blue), roadway slope (α) (Red), pedestrian carry distance (D) (Magenta) and vertical change in height (H)(Green) between takeoff and touchdown on the road surface.

If there is no roadway slope, simply substitute the Red section by using only “g” gravitational acceleration. If there is no significant vertical height difference involved, then you can remove the Green section. If there is no concern for pedestrian and vehicle weights, then remove the Blue section. The same rational would apply to the adjustment for pedestrian carry distance if an adjustment is not required.

NOTE:

During this calculation, it is important to note the roadway slope is reported in degrees and the gravitational acceleration is adjusted first for any roadway slope.


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EXAMPLE: (Considering all situations)

Vehicle Weight (M): 4200 lb (1909.09 kg) Pedestrian Weight (m): 180 lb (81.81 kg) Roadway Slope (α): ± 2 degrees (0.05 %) Throw Distance (s): 125 ft (38.1 m) Carry Distance (D): 3.5 ft (1.06 m) Height Difference (H): 3 ft (0.91 m) Pedestrian Friction (µ): 0.70

( ) ( )( )

min 2

132.2 Cos 2 Sin 2420

2 0.70 125V

1 0.70

0.70 300 180

42.70

5

00

3.⎛ ⎞⎛ ⎞

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⎛ ⎞× −⎜ ⎟⎜ ⎟

⎝ ⎠=+

+− ×

(11)

Using the above scenario, the downhill “-” roadway slope results are 41.6 mph (67.0 km/h) and the uphill “+” roadway slope results are 43.7 mph (70.4 km/h), or a total ± difference of 2.1 mph (3.37 km/h). Taking this one step further, comparing these results to the Searle Minimum Formula with no adjustments, the results are 41.9 mph (67.4 km/h).

If for example the vertical change in height between the pedestrian’s center of mass at takeoff and the center of mass at touchdown is 25 ft (7.62 m), then the downhill “-“ result would be 38.8 mph (62.5 km/h) and the uphill “+” result would yield 40.8 mph (65.7 km/h).

However, since the pedestrian does not acquire 100% of the vehicle’s speed at impact (unless of course this is a forward projection trajectory) the results obtained using the Searle Minimum Formula are conservative and therefore underestimate the vehicle’s striking speed.

It is worthwhile mentioning that investigators may use either Imperial or Metric values in any of the Searle Formulae. The results are reported as “feet per second” for Imperial values, and as “meters per second” units for Metric values.


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CONCLUSIONS On-going crash testing and research shows the pedestrian is carried a short distance before the airborne phase commences. Investigators must be aware that pedestrians may become trapped upon the vehicle, or a secondary contact may occur during the impact phase. Should this occurs, Searle [7] suggests the vehicle’s impact speed could be as much as 80% of the Searle Minimum Formula results. Therefore, it is necessary to consider the effects a secondary contact will have on any overall analysis.

There will be times when investigators are not able to locate the area of impact. In such cases, the only evidence is that of a pedestrian sliding along the road surface. The results obtained from pedestrian sliding will underestimate the vehicle’s impact speed for two reasons: 1) because there is a horizontal loss of projectile speed upon ground impact and 2) because the pedestrian does not acquire 100% of the vehicle’s impact speed, unless the trajectory is a forward projection.

Current crash testing suggests there is a horizontal speed loss of 6.6 mph (10.6 km/h) when the pedestrian first makes ground contact (Appendix B). Additionally, the pedestrian only acquires 78.4% of the vehicle’s impact speed for the bicycle-related crash tests conducted and acquires only 87.3% of the vehicle’s impact speed for all crash tests conducted. Searle’s research [5] found the pedestrian’s combined projection efficiency was 77.5% of the vehicle’s striking speed.

We know from testing that the pedestrian does not always acquire the same percentage of vehicle speed if struck by similar vehicles and under similar impact alignments. Because the projection efficiency, or percentage, varies from test to test, it is not wise for investigators to increase their calculated results to make up for the missing speed percentages. This approach may lead an investigator to falsely overestimate the vehicle’s true speed at impact.

Although there is some pedestrian carry distance before the body becomes airborne, the carry distance is generally short and in the order of 3.2 feet (1 meter) [1, 9]. As a result, the decision to reduce the pedestrian’s total throw distance by a carry distance only has a small effect on the final speed calculations.

If investigators are able to establish a total throw distance from first impact to the pedestrian’s final rest, then the methods provided by Dr. Searle’s research and confirmed by this research, will allow investigators to figure out the speed necessary to project a pedestrian from impact to final rest. The values obtain using this method underestimate the vehicle’s impact speed.

Although there are several factors investigators need to address at some point in their investigations, it appears from this research the effects do not have a significant effect on the overall results. Knowing the pedestrian’s carry distance or the pedestrian’s takeoff angle has very little or no effect on the final calculated results.


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REFERENCES [1] BECKER, T.L. and READE, M.W., Documentation and Analysis of Controlled Pedestrian/

Bicycle Crash Testing, Pedestrian/Bicycle Crash Investigation and Advanced Pedestrian/Bicycle Crash Investigation Courses, Institute of Police Technology and Management, Jacksonville, FL, 2008 - 2011.

[2] RAVANI, B., BROUGHAM, D. and MASON, R.T., Pedestrian Post-Impact Kinematics and Injury Patterns, Traffic Safely Research Corporation, Palo Alto, CA, SAE 811024, 1981.

[3] BECKER, T.L., Vehicle-Pedestrian-Bicycle Collision Investigation Manual, Institute of Police

Technology and Management Publisher, ISBN 1-884566-51-0, pages: 51-52, 2003.

[4] READE, M.W., PEDBIKE 2000 Plus – Pedestrian/Bicycle Specialty Software, Designed and Programmed in Visual Basic 6 by Mike W. Reade, http://frsi.ca/pedbike/pedbike.php

[5] SEARLE, J.A. and SEARLE, A., The Trajectories of Pedestrians, Motorcycles, Motorcyclists, etc., Following a Road Accident, Motor Industry Research Association, SAE 831622, pages: 277-285, 1983.

[6] SEARLE, J.A., The Physics of Throw Distance in Accident Reconstruction, Road Accident Analysis Services, SAE 930659, pages: 71-81, 1993.

[7] SEARLE, J., The Application of Throw Distance Formulae, Road Accident Analysis, Hinckley, UK, Institute of Police Technology and Management, 2009 Special Problems in Traffic Crash Reconstruction Conference, Orlando, FL, 2009.

[8] RESCUE RANDY, The test dummy manikin that was used during the crash testing, Simulaids LLC, http://www.simulaids.com/1475.htm

[9] READE, M.W., How Does Pedestrian Ground Impact and Pedestrian Carry Distance Affect Investigations? A Look at On-Going Testing and Training, Forensic Reconstruction Specialists Inc., Riverview, New Brunswick, Proceedings of the 21st Canadian Multidisciplinary Road Safety Conference, Halifax, Nova Scotia, 2011.

[10] BECKER, T.L. and READE, M.W., A Fresh Approach Into The Reconstruction of Pedestrian/Bicycle Collisions, Institute of Police Technology and Management, 2007 Special Problems Conference, Jacksonville, FL., 2007.

[11] cSwing LLC, Professional Golf Swing Analysis Software, cSwing LLC, El Paso, Texas, http://cswing.com/index.html

[12] DAILY, J., SHIGEMURA, N. and DAILY, J., Fundamentals of Traffic Crash Reconstruction – Volume 2 of the Traffic Crash Reconstruction Series, Institute of Police Technology and Management Publisher, ISBN 978-1-884566-63-9, pages: 491-522, 2007.


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[13] VERICOM PERFORMANCE COMPUTERS, Braking Testing and Acceleration Performance Equipment, Vericom Computers Inc., http://www.vericomcomputers.com/index.html

[14] HILL, G.S., Calculations of Vehicle Speed from Pedestrian Throw, Impact Vol. 4, No. 1, p18-20, ITAI, England, 1994.

[15] CRAIG, A., Bovington Test Results, Impact Vol. 8, No. 8, p83-85, ITAI, England, 1999.

[16] READE, M.W., CATAIR Atlantic Region Pedestrian Crash, Drop & Friction Testing, Riverview, New Brunswick, Canada, 2011.


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Appendix A - Schematic & Terms of a Typical Pedestrian Throw Event

Terminology: d = Sliding distance of pedestrian along ground surface, or vehicle braking distance

D = Carry distance of pedestrian from impact to start of airborne phase

µ = Coefficient of friction of sliding pedestrian along the ground surface

f = Coefficient of friction of braking vehicle along the roadway

g = Gravitational acceleration

s = Total pedestrian throw distance from impact to final rest

Vmin = Projectile velocity using Searle minimum formula

vo = Projectile’s original velocity on takeoff (Same result as Vmin)

θ = Projectile’s takeoff angle in degrees

v = Projectile’s vertical velocity on takeoff

H = Vertical change in height of center of mass between takeoff and touchdown

V = Horizontal velocity loss on landing

H

d

v

Airborne Distance

s (Total Throw Distance)

θTouchdown

Final Rest

Projectile C/M Travel Path

D


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Appendix B – Summary of Crash Test Data


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the anatomy and analysis of a typical pedestrian / bicycle ... · the anatomy and analysis of a...

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