Chapter 1260 Sight Distance Design Manual M 22 01 2
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Chapter 1260 1260.01 1260.02 1260.03 1260.04 1260.05 1260.06 Sight Distance General References Stopping Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft) Passing Sight Distance (Eye height – 3.5 ft, Object height – 3.5 ft) Decision Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft) Documentation 1260.01 General Sight distance allows the driver to assess developing situations and take actions appropriate for the conditions. Sight distance relies on drivers being aware of and paying attention to their surroundings and driving appropriately for conditions presented. For the purposes of design, sight distance is considered in terms of stopping sight distance, passing sight distance, and decision sight distance. For additional information, see the following chapters: Chapter 1250 1310 1320 1340 1515 Subject Sight distance at railroad crossings Sight distance at intersections at grade Sight distance at roundabouts Sight distance at driveways Sight distance for shared-use paths 1260.02 References 1260.02(1) Design Guidance Manual on Uniform Traffic Control Devices for Streets and Highways, USDOT, FHWA; as adopted and modified by Chapter 468-95 WAC “Manual on uniform traffic control devices for streets and highways” (MUTCD) 1260.02(2) Supporting Information A Policy on Geometric Design of Highways and Streets (Green Book), AASHTO Passing Sight Distance Criteria, NCHRP 605 1260.03 1260.03(1) Stopping Sight Distance (Eye height – 3.5 ft, Object height – 2.0 ft) Design Criteria Stopping sight distance is provided when the sight distance available to a driver equals or exceeds the stopping distance for a passenger car traveling at the design speed. Stopping distance for design is very conservatively calculated, with lower deceleration and slower perception reaction time than normally expected from the driver. Provide design stopping sight distance at all points on all highways and on all intersecting roadways, unless a design analysis is deemed appropriate. WSDOT Design Manual M 22-01.13 July 2016 Page 1260-1 Sight Distance 1260.03(1)(a) Chapter 1260 Stopping Sight Distance Stopping sight distance is the sum of two distances: the distance traveled during perception and reaction time and the distance to stop the vehicle. The perception and reaction distance used in design is the distance traveled in 2.5 seconds at the design speed. The design stopping sight distance is calculated using the design speed and a constant deceleration rate of 11.2 feet/second2. For stopping sight distances on grades less than 3%, see Exhibit 1260-1; for grades 3% or greater, see Exhibit 1260-2. 1260.03(1)(b) Design Stopping Sight Distance Exhibit 1260-1 gives the design stopping sight distances for grades less than 3%, the minimum curve length for a 1% grade change to provide the stopping sight distance for a crest (Kc) and sag (Ks) vertical curve, and the minimum length of vertical curve for the design speed (VCLm). For stopping sight distances when the grade is 3% or greater, see Exhibit 1260-2. Design Speed (mph) Design Stopping Sight Distance (ft) Kc Ks VCLm (ft) 25 30 35 40 45 50 55 60 65 70 75 80 155 200 250 305 360 425 495 570 645 730 820 910 12 19 29 44 61 84 114 151 193 247 312 384 26 37 49 64 79 96 115 136 157 181 206 231 75 90 105 120 135 150 165 180 195 210 225 240 Design Stopping Sight Distance Exhibit 1260-1 Page 1260-2 WSDOT Design Manual M 22-01.10 July 2013 Chapter 1260 Sight Distance 1260.03(2) Effects of Grade The grade of the highway has an effect on the stopping sight distance. The stopping distance is increased on downgrades and decreased on upgrades. Exhibit 1260-2 gives the stopping sight distances for grades of 3% and steeper. When evaluating sight distance with a changing grade, use the grade for which the longest sight distance is needed. Design Speed (mph) 25 30 35 40 45 50 55 60 65 70 75 80 Stopping Sight Distance (ft) Downgrade Upgrade -3% -6% -9% 3% 6% 9% 158 205 257 315 378 446 520 598 682 771 866 965 140 179 222 269 320 375 433 495 561 631 704 782 165 215 271 333 400 474 553 638 728 825 927 1,035 173 227 287 354 427 507 593 686 785 891 1,003 1,121 147 200 237 289 344 405 469 538 612 690 772 859 143 184 229 278 331 388 450 515 584 658 736 817 Design Stopping Sight Distance on Grades Exhibit 1260-2 For stopping sight distances on grades between those listed, interpolate between the values given or use the equation in Exhibit 1260-3. S = 1.47V (2.5) + V2 G 30 0.347826 ± 100 Where: S = Stopping sight distance on grade (ft) V = Design speed (mph) G = Grade (%) Stopping Sight Distance on Grades Exhibit 1260-3 1260.03(3) Crest Vertical Curves When evaluating an existing roadway, refer to 1260.03(7). Use Exhibit 1260-4 or the equations in Exhibit 1260-5 to find the minimum crest vertical curve length to provide stopping sight distance when given the algebraic difference in grades. Exhibit 1260-4 does not show the sight distance greater than the length of curve equation. WSDOT Design Manual M 22-01.10 July 2013 Page 1260-3 Sight Distance Chapter 1260 When the sight distance is greater than the length of curve and the length of curve is critical, the S>L equation given in Exhibit 1260-5 shall be used to find the minimum curve length. When a new crest vertical curve is built or an existing one is rebuilt with grades less than 3%, provide design stopping sight distance from Exhibit 1260-1. For grades 3% or greater, provide stopping sight distance from 1260.03(2). S Object height h2=2.0' Eye height h1=3.5' 0f S= 30 , ph 50 45 mp h, S= h, mp 40 5 42 36 5f 50 f S=2 ft. 35 m ph, =200 . ph, S S=155 ft 12 30 m 25 mph, Algebraic Difference in Grade, A (%) 14 t. t. 16 t. The minimum length can also be determined by multiplying the algebraic difference in grades by the KC value from Exhibit 1260-1 (L=KC*A). Both the exhibit and the equation give approximately the same length of curve. Neither use the S>L equation. ft. 5 49 ft. = S= ,S ph m 55 t. 0f m h, p 0m 7 =5 S 6 . 5 ft ph, 64 S= 10 m 65 8 70 . 0 ft 73 , S= h mp . 20 ft , S=8 h p . 75 m 10 ft , S=9 h p 80 m 6 4 2 0 0 500 1500 1000 2000 Length of Vertical Curve, L (ft) Stopping Sight Distance: Crest Vertical Curves Exhibit 1260-4 Page 1260-4 WSDOT Design Manual M 22-01.10 July 2013 Chapter 1260 Sight Distance When S>L 2158 L+ A S= 2 2158 L = 2S − A When SL equation given in Exhibit 1260-7 shall be used to find the minimum length of curve. When a new sag vertical curve is built or an existing one is rebuilt with grades less than 3%, provide design stopping sight distance from Exhibit 1260-1. For grades 3% or greater, provide stopping sight distance from 1260.03(2). WSDOT Design Manual M 22-01.10 July 2013 Page 1260-5 Sight Distance Chapter 1260 S 1° 2.0' ft =3 05 ph m 45 ft 95 ,S ,S =3 ,S ph m 40 5 2 =4 60 ft t 0f 25 S= h, mp 35 mp 30 12 h, S S=1 25 m ph, 14 Algebraic Difference in Grade, A (%) =20 55 f t 16 0 ft The minimum length can also be determined by multiplying the algebraic difference in grades by the KS value from Exhibit 1260-1 (L=KS*A). Both the exhibit and equation give approximately the same length of curve. Neither use the S>L equation. ph 50 m , ph ft 4 S= m 55 t 0f 57 = ,S h 60 mp h, 65 mp 5 ft 64 S= ft 30 S=7 , ft h mp S=820 70 , h t mp 10 f 75 S=9 , h p 80 m 10 8 6 4 2 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Length of Curve, L (ft) Stopping Sight Distance for Sag Vertical Curves Exhibit 1260-6 Page 1260-6 WSDOT Design Manual M 22-01.10 July 2013 Chapter 1260 Sight Distance When S>L L = 2S − 400 + 3.5S A S= LA + 400 2 A − 3.5 When S L L = 2S − 2800 A S= L 1400 + 2 A S= 2800 L A When S
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