* Speed category to be determined by the highway agency, ** The column headings A, B, and C are the dimensions shown in Figures 6H-1 through 6H-46. You are shown an accident scene with a vehicle and a tree on uphill grade of 3%. The C dimension is the distance between the second and third signs. to see an object on the roadway ahead and bring their vehicles to safe Traffic control planning should be completed for all highway construction, utility work, maintenance operations, and incident management including minor maintenance and utility projects prior to occupying the TTC zone. 13 When a shadow vehicle, arrow board, or changeable message sign is placed in a closed lane in advance of a work space, only the area upstream of the vehicle, arrow board, or changeable message sign constitutes the buffer space. The overtaking sight distance or passing sight distance is measured along the center line of the road over which a driver with his eye level 1.2 m above the road surface can see the top of an object 1.2 m above the road surface. However, there are cases where it may not be appropriate. Standard: Stopping Sight Distance (SSD) is the viewable distance required for a driver to see so that he or she can make a complete stop in the event of an unforeseen hazard. It extends from the first warning device (such as a sign, light, or cone) to the last TTC device or to a point where road users return to the original lane alignment and are clear of the incident. Option: gEZd|t->gzL5G(7V=^|z~PS+f|0@+ms,_7ZT4qO/=H+8}\9z&KUk>SrVT#5$3m8'iLIj 7-3G&?$4> distance are the same in terms of safety risk. 04 The appropriate taper length (L) should be determined using the criteria shown in Tables 6C-3 and 6C-4. Federal Highway Administration Research has demonstrated that large reductions in the speed limit, such as a 30 mph reduction, increase speed variance and the potential for crashes. 03 An incident zone is an area of a highway where temporary traffic controls are imposed by authorized officials in response to a traffic incident (see Section 6I.01). Option: 03 Typical distances for placement of advance warning signs on freeways and expressways should be longer because drivers are conditioned to uninterrupted flow. 19). 12 Typically, the buffer space is formed as a traffic island and defined by channelizing devices. How do the calculations of stopping sight distance and passing sight distance differ? Support: In these instances, the same type, but abbreviated, closure procedures used on a normal portion of the roadway can be used. Not too often, more often taken as a function of materials and construction, and wear and tear on road (older roads have less friction). This AASHTO formula is used in road design for establishing the minimum stopping sight distance. Reduced speed zoning (lowering the regulatory speed limit) should be avoided as much as practical because drivers will reduce their speeds only if they clearly perceive a need to do so. A short taper having a minimum length of 50 feet and a maximum length of 100 feet with channelizing devices at approximately 20-foot spacing should be used to guide traffic into the one-lane section, and a downstream taper with a length of 100 feet should be used to guide traffic back into their original lane. Book provides design criteria for decision sight distance, passing of a design exception for stopping sight distance. Stopping sight distance is influenced by both vertical and horizontal alignment. 05 Traffic control planning should be completed for all highway construction, utility work, maintenance operations, and incident management including minor maintenance and utility projects prior to occupying the TTC zone. 02 An END ROAD WORK sign, a Speed Limit sign, or other signs may be used to inform road users that they can resume normal operations. The second distance component \(d_2\) is defined as: The third distance component \(d_3\) is more of a rule of thumb than a calculation. 4. sight distance cannot be provided. Design Speed (mph) Stopping Sight Distance (ft) 15 80 20 115 25 155 30 200 35 250 40 305 45 360 50 425 55 495 60 570 65 645 70 730 75 820 80 910 The main difference between the DSD and SSD criteria is the complexity of the situation that the driver is faced with. You have found that a car traveling that section under similar weather conditions at 60 km*h1 can stop in 60 m. What was its initial speed? What happens if an accident is caused by poorly designed roads? The forces acting this vehicle can be simplified to: Using Newtons second law we can conclude then that the acceleration (\(a\)) of the object is, Using our basic equations to solve for braking distance (\(d_b\)) in terms of initial speed (\(v_i\)) and ending speed (\(v_e\)) gives, and substituting for the acceleration yields, \[d_b=\frac{v_i^2-v_e^2}{2g(fcos(\theta)-sin(\theta))}\], For angles commonly encountered on roads, \(cos(\theta) \approx 1\) and \(sin(\theta) \approx tan(\theta)=G\), where \(G\) is called the roads grade. Guidance: When two or more advance warning signs are used on higher-speed streets, such as major arterials, the advance warning area should extend a greater distance (see, Since rural highways are normally characterized by higher speeds, the effective placement of the first warning sign in feet should be substantially longerfrom 8 to 12 times the speed limit in mph. A short taper having a minimum length of 50 feet and a maximum length of 100 feet with channelizing devices at approximately 20-foot spacing should be used to guide traffic into the one-lane section, and a downstream taper with a length of 100 feet should be used to guide traffic back into their original lane. Examples include bridge piers, barrier, walls, profile (stopping sight distance as a function of distance along the roadway). sight distance (Figure 17). Horizontal Sightline Offset The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. ability of most vehicles under wet pavement conditions, and the friction The speed limit should be stepped down in advance of the location requiring the lowest speed, and additional TTC warning devices should be used. k!lA/CtO^b2O"3?b1iDS6 SDbjcHy_C-} 7txV^xQgUhl)tW 4kl9R)2MC4g9-?zl,9k`zY This Page Intentionally Left Blank. Types of tapers are shown in Figure 6C-2. Should be on average correct . The termination area extends from the downstream end of the work area to the last TTC device such as END ROAD WORK signs, if posted. understand the severity of a sight distance restriction, how the restriction 16 An example of a one-lane, two-way traffic taper is shown in Figure 6C-3. This information can help designers What is the traffic volume through the location with limited sight distance. The "third sign" is the sign that is furthest upstream from the TTC zone.). A pilot car may be used to guide a queue of vehicles through the TTC zone or detour. Lawsuits, Explain Stopping Sight Distance again = perception reaction distance + braking distance. 01 The driver of the last vehicle proceeding into the one-lane section is given a red flag (or other token) and instructed to deliver it to the flagger at the other end. When redirection of the road users' normal path is required, they shall be directed from the normal path to a new path. are nearly equal. the top photo, a car is visible as it approaches the crest of a hill. Exhibit 1 Stopping Sight Distance (2011 AASHTO Table 3-1, 3-4) Horizontal Stopping Sight Distance "Another element of horizontal alignment is the sight distance across the inside of curves (often referred to as Horizontal Sightline Offset. If lighting is provided at sag vertical curves, a design A vehicle initially traveling at 150 km/hr skids to a stop on a 3% downgrade, taking 200 m to do so. or local). >Ll=fDH#Rh B:('$EQxG= 4VI3LU.UuO*]ZGwAswD\+^ XFJ]g~Z&zV%<7MqJ :/6&8|y2 yvs2K`BId>L4ILrN $*;OT;QOz&h\wZS (!naM a lower coefficient of friction. Since there might be several work spaces (some even separated by several kilometers or miles) within the project limits, each work space should be adequately signed to inform road users and reduce confusion. Forces acting on a vehicle that is braking. tables are based on the AASHTO's "A Policy on Geometric Design of Highways and Streets," 2011. bottom photo shows the actual curve in the road, and the lack of connection These manual signals should not be mistaken for flagging signals. 3jTCyw$u0J;H}/q@#(C 15 Traffic should be controlled by a flagger or temporary traffic control signal (if sight distance is limited), or a STOP or YIELD sign. This alternate or modified plan should have the approval of the responsible highway agency prior to implementation. Support: The first distance component \(d_1\) is defined as: \[d_1=1000t_1 \left( u-m+\frac{at_1}{2} \right)\]. 5.1.1.2 Decision Sight Distances (DSDs) We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. What can stopping distance measure be used for? 02 Detours should be clearly signed over their entire length so that road users can easily use existing highways to return to the original highway. Standard: with limited sight distance involves the following questions: For example, the risk associated with a crest vertical curve with non-standard A diversion is a temporary rerouting of road users onto a temporary highway or alignment placed around the work area. less. 01 A TTC plan describes TTC measures to be used for facilitating road users through a work zone or an incident area. Barriers and channelizing devices that are detectable by people with visual disabilities should be provided. 2. Therefore, the advance warning sign placement should extend on these facilities as far as 1/2 mile or more. \[d_s=d_r+d_b=0.278t_rv_i+\frac{(0.278v_i)^2}{19.6(f \pm G)}\], We said \(d_b=\frac{v_i^2-v_e^2}{2g(f \pm G)\), Use: \((f-G)\) if going downhill and \((f+G)\) if going uphill, where G is the absolute value of the grade. 06/28/2019. entire facility. Karen Dixon and her students, source@https://en.wikibooks.org/wiki/Fundamentals_of_Transportation, status page at https://status.libretexts.org. 03 The work space may be stationary or may move as work progresses. Forces acting on a vehicle that is braking TTC plans range in scope from being very detailed to simply referencing typical drawings contained in this Manual, standard approved highway agency drawings and manuals, or specific drawings contained in the contract documents. Guidance: A simple model for evaluating locations is influenced by both vertical and horizontal alignment, a design exception Guidance: A detour is a temporary rerouting of road users onto an existing highway in order to avoid a TTC zone. In the second photo, the back of the car is seen descending the far side endobj Changes in alignment can also be accomplished by using horizontal curves designed for normal highway speeds. Whenever tapers are to be used in close proximity to an interchange ramp, crossroads, curves, or other influencing factors, the length of the tapers may be adjusted. Reduced speed limits should be used only in the specific portion of the TTC zone where conditions or restrictive features are present. Whenever tapers are to be used in close proximity to an interchange ramp, crossroads, curves, or other influencing factors, the length of the tapers may be adjusted. Support: This distance also allows the driver to abort the passing maneuver if desired. S = sight distance in ft, PVC = point of the vertical curve (the initial point of the . Provisions for effective continuity of railroad service and acceptable access to abutting property owners and businesses should also be incorporated into the TTC planning process. Horizontal Sightline Offset The test concerning adequate lengths of tapers involves observation of driver performance after TTC plans are put into effect. 01 The transition area is that section of highway where road users are redirected out of their normal path. 1. 01 STOP or YIELD signs may be used to control traffic on low-volume roads at a one-lane, two-way TTC zone when drivers are able to see the other end of the one-lane, two-way operation and have sufficient visibility of approaching vehicles. sight distance (applies to two-lane roads only) and intersection sight Smaller reductions in the speed limit of up to 10 mph cause smaller changes in speed variance and lessen the potential for increased crashes. 01 The advance warning area is the section of highway where road users are informed about the upcoming work zone or incident area. 03 Because it is impractical in mobile operations to redirect the road user's normal path with stationary channelization, more dominant vehicle-mounted traffic control devices, such as arrow boards, portable changeable message signs, and high-intensity rotating, flashing, oscillating, or strobe lights, may be used instead of channelizing devices to establish a transition area. What does a negative grade mean if you are were assuming to be going downhill? Support: 4 0 obj Stopping sight distance for passenger vehicles on horizontal . Name five principal characteristics of visual reception important in driving. 4. In computing and measuring stopping sight distance, the height of the driver's eye is estimated to be 3.5-ft and the height of the object to be seen by the driver is 2.0-ft, equivalent to the taillight height of passenger car. Figure 6C-1 Component Parts of a Temporary Traffic Control Zone. TTC plans should be prepared by persons knowledgeable (for example, trained and/or certified) about the fundamental principles of TTC and work activities to be performed. For sag vertical curves, formal design exceptions are required for curves Guidance: Support: If a longitudinal buffer space is used, the values shown in Table 6C-2 may be used to determine the length of the longitudinal buffer space. This model has been altered only slightly since its inception . Control points at each end should be chosen to permit easy passing of opposing lanes of vehicles. A 06 Provisions for effective continuity of accessible circulation paths for pedestrians should be incorporated into the TTC process. Since two or more advance warning signs are normally used for these conditions, the advance warning area should extend 1,500 feet or more for open highway conditions (see Table 6C-1). A longitudinal buffer space may be used between the work space and the beginning of the downstream taper. 3 0 obj Perform sight distance analysis. 08 Modifications of TTC plans may be necessary because of changed conditions or a determination of better methods of safely and efficiently handling road users. A downstream taper might be useful in termination areas to provide a visual cue to the driver that access is available back into the original lane or path that was closed. speed range results in minimum curve lengths of about half those based Table 17 summarizes the potential adverse impacts to safety and operations In addition to stopping sight distance, the Green Washington, DC. 07 A merging taper should be long enough to enable merging drivers to have adequate advance warning and sufficient length to adjust their speeds and merge into an adjacent lane before the downstream end of the transition. Option: Option: The maximum distance in feet between devices in a taper should not exceed 1.0 times the speed limit in mph. The duration of the TTC zone is determined by the duration of the planned special event. 09 A shifting taper should have a length of approximately 1/2 L (see Tables 6C-3 and 6C-4). A work zone is an area of a highway with construction, maintenance, or utility work activities. 202-366-4000, FHWA Home / 04 Coordination should be made between adjacent or overlapping projects to check that duplicate signing is not used and to check compatibility of traffic control between adjacent or overlapping projects. Important auxiliary provisions that cannot conveniently be specified on project plans can easily be incorporated into Special Provisions within the TTC plan. 07 The need to provide additional reaction time for a condition is one example of justification for increasing the sign spacing. compared with a similar location with no such features. 02 When redirection of the road users' normal path is required, they shall be directed from the normal path to a new path. How significant is the deficiency in sight distance (as measured by length of highway as well as amount of deficiency relative to that required per adopted criteria)? In this example, 2. An incident zone is an area of a highway where temporary traffic controls are imposed by authorized officials in response to a traffic incident (see. Passing Sight Distance (PSD) is the minimum sight distance that is required on a highway, generally a two-lane, two-directional one, that will allow a driver to pass another vehicle without colliding with a vehicle in the opposing lane. Guidance: Figure 19 is a photo showing a truck about to pass underneath an overhead Support: What are the steps in accident reconstruction. A shifting taper should have a length of approximately 1/2 L (see. Mitigation Strategies For Design Exceptions. The activity area may contain one or more lateral or longitudinal buffer spaces. Support: 14 The lateral buffer space may be used to separate the traffic space from the work space, as shown in Figures 6C-1 and 6C-2, or such areas as excavations or pavement-edge drop-offs. 01 A TTC plan describes TTC measures to be used for facilitating road users through a work zone or an incident area. Table 16 This paper presents the concept and analysis of the first three types of sight distance based on AASHTO models: 1) the sight distances needed for stopping, applicable to all highway travels; 2) the sight distances needed for decisions at hazardous complex locations; and 3) the passing sight distance needed on two lane highways. Support: Vertical stopping sight distance at a crest STOP or YIELD signs may be used to control traffic on low-volume roads at a one-lane, two-way TTC zone when drivers are able to see the other end of the one-lane, two-way operation and have sufficient visibility of approaching vehicles. Expressway: high-speed, multi-lane divided arterial with interchange refer to HDM Chapter 7, Exhibit 7-7 Minimum Stopping Sight Distance (SSD). Exhibit 1 Stopping Sight Distance (2011 AASHTO Table 3-1, 3-4) Horizontal Stopping Sight Distance "Another element of horizontal alignment is the sight distance across the inside of curves (often referred to as Horizontal Sightline Offset. Guidance: Provisions for effective continuity of accessible circulation paths for pedestrians should be incorporated into the TTC process. endobj a curved portion of road. A reduction of more than 10 mph in the speed limit should be used only when required by restrictive features in the TTC zone. The termination area is the section of the highway where road users are returned to their normal driving path. the intersecting roadway in the background creates the illusion of a straight The PILOT CAR FOLLOW ME (G20-4) sign (see. The test concerning adequate lengths of tapers involves observation of driver performance after TTC plans are put into effect. Publications / Geometric Design / Because stopping sight distance The duration of the TTC zone is determined by the duration of the planned special event. to implement mitigation strategies. The activity area may contain one or more lateral or longitudinal buffer spaces. The AASHTO stopping distance formula is as follows: s = (0.278 t v) + v / (254 (f + G)) where: s - Stopping distance in meters; t - Perception-reaction time in seconds; v - Speed of the car in km/h; G - Grade (slope) of the road, expressed as a decimal. lighting is provided. \(d_b=\frac{\left( 150* (\frac{1000}{3600}) \right)^2-(0)^2}{2*(9.8)*(0.40-G)}=200m\), \((0.40-G)=\frac{\left( 150* (\frac{1000}{3600}) \right)^2-(0)^2}{2*(9.8)*200}\). 11 If a longitudinal buffer space is used, the values shown in Table 6C-2 may be used to determine the length of the longitudinal buffer space. When two or more advance warning signs are used on higher-speed streets, such as major arterials, the advance warning area should extend a greater distance (see Table 6C-1). How fast was the vehicle traveling to begin with? Guidance: Support: You are shown an crash scene with a vehicle and a light pole. 03 A longitudinal buffer space may be used between the work space and the beginning of the downstream taper. Access to temporary bus stops, travel across intersections with accessible pedestrian signals (see Section 4E.09), and other routing issues should be considered where temporary pedestrian routes are channelized. However, frequent changes in the speed limit should be avoided. <> It extends from the first warning sign or high-intensity rotating, flashing, oscillating, or strobe lights on a vehicle to the END ROAD WORK sign or the last TTC device. Option: (Wikipedia) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request. In A trial run that is conducted to help measure the coefficient of friction reveals that a car traveling at 60 km/hr can stop in 100 meters under conditions present at the time of the accident. When a shadow vehicle, arrow board, or changeable message sign is placed in a closed lane in advance of a work space, only the area upstream of the vehicle, arrow board, or changeable message sign constitutes the buffer space. Transition areas usually involve strategic use of tapers, which because of their importance are discussed separately in detail. Rural 2-Lane: high-speed, undivided rural highway (arterial, collector, and other roadway features with non-standard stopping sight distance. sight distance profile allows a designer to identify the region of minimum 03 TTC plans should be prepared by persons knowledgeable (for example, trained and/or certified) about the fundamental principles of TTC and work activities to be performed. Stopping Sight Distance (2004 AASHTO Exhibit 3-1, 112) Horizontal Stopping Sight Distance "Another element of horizontal alignment is the continuous sight distance available across the inside of curves, often referred to as Horizontal Sightline Offset. ~\V}I*0(=C!dH#B8^gOnX86yHXz>Qm|Tu):8RHPUr&JRkL(CzpAhQ43dELu{}C1U"XSa:t`,oRQ?j3[8QPn{p_8% P2wlMHb F6$m2N*c)ad aoLMXR#ki:t: F44Wl]G:@VG 01 Except as provided in Paragraph 5, when traffic in both directions must use a single lane for a limited distance, movements from each end shall be coordinated. Stopping Sight Distance. Issued by FHWA, S = posted speed limit, or off-peak 85th-percentile speed prior to work starting, or the anticipated operating speed in mph. For stopping distance, why dont we have a factor to include vehicle size and weight? The 03 The PILOT CAR FOLLOW ME (G20-4) sign (see Section 6F.58) shall be mounted on the rear of the pilot vehicle. Isnt 200 m long distance for braking Yes unless very high speed on very slick surface (or going downhill). For crest vertical curves, the alignment of the roadway limits stopping 2 0 obj AASHTO, 2018, Pages 3-1 thru 3-19, Chapter 3 Elements of Design, Section 3.2 Sight Distance . or other roadway features (Figure 21) within the area of the sight restriction Transition areas usually involve strategic use of tapers, which because of their importance are discussed separately in detail. How far does the vehicle travel before coming to a stop? The unit conversions convert the problem to metric, with \(v_i\) in kilometers per hour and \(d_s\) in meters. The The work space may be stationary or may move as work progresses. \5:,nzx_c*&%G7qE?; +A*Q84#4 (ii8Yu p1rYOU>M1]{diGqBR"dJQgoW/62a- 0007eRB1b 1200 New Jersey Avenue, SEWashington, DC 20590 at crest vertical curves (Figure 18), headlight sight distance at sag 01 Traffic control signals may be used to control vehicular traffic movements in one-lane, two-way TTC zones (see Figure 6H-12 and Chapter 4H). A reduction in the regulatory speed limit of only up to 10 mph from the normal speed limit has been shown to be more effective. If a shoulder is used as a travel lane, either through practice or during a TTC activity, a normal merging or shifting taper should be used. 2. stop before colliding with the object. \(m\) = difference in speeds of passing and impeder vehicles (km/hr). 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