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### Stopping Distance by Sight Calculator and Formulas

Car Stopping Distance by Sight Calculator SSD

A driver’s ability to see ahead is needed for safe and efficient operation of a vehicle on a highway. For example, on a railroad, trains are confined to a fixed path, yet a block signal system and trained operators are needed for safe operation. In contrast, the path and speed of motor vehicles on highways and streets are subject to the control of drivers whose ability, training, and experience are quite varied.

Sight distance is the length of the roadway ahead that is visible to the driver. The available sight distance on a roadway should be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path.

Stopping sight distance is the sum of two distances: (1) the distance traversed by the vehicle from the instant the driver sights an object necessitating a stop to the instant the brakes are applied, and (2) the distance needed to stop the vehicle from the instant brake application begins. These are referred to as brake reaction distance and braking distance, respectively.

The stopping sight distance is the sum of the distance traversed during the brake reaction time and the distance to brake the vehicle to a stop. The computed distances for various speeds at the assumed conditions on level roadways To calculate SSD

Stopping sight distance on level road

Eq. 1
Imperial Units
SSD = 1.47 V t + ( 1.075 V2 ) / a

where

SSD = stopping distance, ft
V = design or initial speed, mph
t = brake reaction time, typically about 2.5 s
a = deceleration rate, ft/s2 see note 1

Eq. 2
Metric SI Units
SSD = 0.278 V t + ( 0.039 V2 ) / a

where:

SSD = stopping sight distance, m
V = design or initial speed, km/h
t = brake reaction time, 2.5 s
a = deceleration rate, m/s2 see note 1

When a highway is on a grade, Equations 1 and 2 for braking distance is modified as follows:

Eq. 3
Imperial Units
dB = 1.47 V t + V2 / { 30 [ ( a / 32.2 ) ± G ] }

where:

dB = braking distance on grade, ft
V =design or initial speed, mph
a = deceleration, ft/s2 see note 1

Eq. 4
Metric SI Units
dB = 0.278 V t + V2 / { 254 [ ( a / 9.81 ) ± G ] }

where:
dB = braking distance on grade, m
V = design or initial speed, km/h
a = deceleration, m/s2 see note 1

In these equations, G is the rise in elevation divided by the distance of the run and the percent of grade divided by 100, and the other terms are as previously stated. The stopping distances needed on upgrades are shorter than on level roadways; those on downgrades are longer. The stopping sight distances for various grades shown in the Tables below are the values determined by using Equations 3 and 4 in place of the second term in Equations 1 and 2. These adjusted sight distance values are computed for wet-pavement conditions using the same design speeds and brake reaction times used for level roadways.

 Table 1 U.S. Customary Stopping Sight Distance on Level Roadways Design Speed (mph) Brake Reaction Distance (ft) Braking Distance on Level (ft) Stopping Sight Distance Calculated (ft) Design (ft) 15 55.1 21.6 76.7 80 20 73.5 38.4 111.9 115 25 91.9 60.0 151.9 155 30 110.3 86.4 196.7 200 35 128.6 117.6 246.2 250 40 147.0 153.6 300.6 305 45 165.4 194.4 359.8 360 50 183.8 240.0 423.8 425 55 202.1 290.3 492.4 495 60 220.5 345.5 566.0 570 65 238.9 405.5 644.4 645 70 257.3 470.3 727.6 730 75 275.6 539.9 815.5 820 80 294.0 614.3 908.3 910 85 313.5 693.5 1007.0 1010

 Table 2 Metric Stopping Sight Distance on Level Roadways Design Speed (km/h) Brake Reaction Distance (m) Braking Distance on Level (m) Stopping Sight Distance Calculated (m) Design (m) 20 13.9 4.6 18.5 20 30 20.9 10.3 31.2 35 40 27.8 18.4 46.2 50 50 34.8 28.7 63.5 65 60 41.7 41.3 83.0 85 70 48.7 56.2 104.9 105 80 55.6 73.4 129.0 130 90 62.6 92.9 155.5 160 100 69.5 114.7 184.2 185 110 76.5 138.8 215.3 220 120 83.4 165.2 248.6 250 130 90.4 193.8 284.2 285 140 97.3 224.8 322.1 325

Note: Brake reaction distance predicated on a time of 2.5 s; deceleration rate of 11.2 ft/s2 [3.4 m/s2] used to determine calculated sight distance.

 Table 3 U.S. Customary Stopping Sight Distance on Grades Design Speed (mph) Stopping Sight Distance (ft) Downgrades Upgrades 3% 6% 9% 3% 6% 9% 15 80 82 85 75 74 73 20 116 120 126 109 107 104 25 158 165 173 147 143 140 30 205 215 227 200 184 179 35 257 271 287 237 229 222 40 315 333 354 289 278 269 45 378 400 427 344 331 320 50 446 474 507 405 388 375 55 520 553 593 469 450 433 60 598 638 686 538 515 495 65 682 728 785 612 584 561 70 771 825 891 690 658 631 75 866 927 1003 772 736 704 80 965 1035 1121 859 817 782 85 1070 1149 1246 949 902 862

 Table 4 Metric Stopping Sight Distance on Grades Design Speed (mph) Stopping Sight Distance (m) Downgrades Upgrades 3% 6% 9% 3% 6% 9% 20 20 20 20 19 18 18 30 32 35 35 31 30 29 40 50 50 53 45 44 43 50 66 70 74 61 59 58 60 87 92 97 80 77 75 70 110 116 124 100 97 93 80 136 144 154 123 118 114 90 164 174 187 148 141 136 100 194 207 223 174 167 160 110 227 243 262 203 194 186 120 263 281 304 234 223 214 130 302 323 350 267 254 243 140 341 367 398 302 287 274

Notes:

1. Studies documented have shown that most drivers decelerate at a rate greater than 14.8 ft/s2 [4.5 m/s2] when confronted with the need to stop for an unexpected object in the roadway. Approximately 90 percent of all drivers decelerate at rates greater than 11.2 ft/s2 [3.4 m/s2]. Such decelerations are within the driver’s capability to stay within his or her lane and maintain steering control during the braking maneuver on wet surfaces. Therefore, 11.2 ft/s2 [3.4 m/s2] (a comfortable deceleration for most drivers) is recommended as the deceleration threshold for determining stopping sight distance. Implicit in the choice of this deceleration threshold is the assessment that most vehicle braking systems and the tire-pavement friction levels of most roadways are capable of providing a deceleration rate of at least 11.2 ft/s2 [3.4 m/s2]. The friction available on most wet pavement surfaces and the capabilities of most vehicle braking systems can provide braking friction that exceeds this deceleration rate.
2. Brake Reaction Time. This is the time interval between when an obstacle in the road can first be physically seen and when the driver first applies the brakes. The assumed value is 2.5 s. This time is considered adequate for 90% of drivers in simple to moderatelycomplex highway environments.
3. Speed. The SSD tables provide a minimum value which is based on the design speed.
4. Grade Adjustment. AASHTO 's A Policy on Geometric Design of Highways and Streets provides values to adjust the SSD for each grade which, theoretically, affects braking distances. Due to the conservative SSD model and the nature of the State's terrain, the use of the grade adjustment is not required.

Related:

Source

AASHTO A policy on Geopmetric Design of Highways and Streets, 2018 7th Edition