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Stopping Distance by Sight Calculator and Formulas
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Power Transmission Design and Engineering
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 V^{2} ) / 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/s^{2} see note 1
Eq. 2
Metric SI Units
SSD = 0.278 V t + ( 0.039 V^{2} ) / 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/s^{2} see note 1
Effect of Grade on Stopping
When a highway is on a grade, Equations 1 and 2 for braking distance is modified as follows:
Eq. 3
Imperial Units
d_{B} = 1.47 V t + V^{2} / { 30 [ ( a / 32.2 ) ± G ] }
where:
d_{B} = braking distance on grade, ft
V =design or initial speed, mph
a = deceleration, ft/s^{2} see note 1
G = grade, rise/run, ft/ft
Eq. 4
Metric SI Units
d_{B} = 0.278 V t + V^{2} / { 254 [ ( a / 9.81 ) ± G ] }
where:
d_{B} = braking distance on grade, m
V = design or initial speed, km/h
a = deceleration, m/s^{2} see note 1
G = grade, rise/run, m/m
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 wetpavement 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/s^{2} [3.4 m/s^{2}] 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:
 Studies documented have shown that most drivers decelerate at a rate greater than 14.8 ft/s^{2} [4.5 m/s^{2}] 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/s^{2} [3.4 m/s^{2}]. 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/s^{2} [3.4 m/s^{2}] (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 tirepavement friction levels of most roadways are capable of providing a deceleration rate of at least 11.2 ft/s^{2} [3.4 m/s^{2}]. 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.
 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.
 Speed. The SSD tables provide a minimum value which is based on the design speed.
 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.
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Source
AASHTO A policy on Geopmetric Design of Highways and Streets, 2018 7th Edition