Section 3: Sight Distance
Anchor: #i1085655Overview
This section provides descriptions and information on sight distance, one of several principal elements of design that are common to all types of highways and streets. Of utmost importance in highway design is the arrangement of geometric elements so that there is adequate sight distance for safe and efficient traffic operation assuming adequate light, clear atmospheric conditions, and drivers' visual acuity. For design, the following four types of sight distance are considered:
Anchor: #CHDCDCCHStopping Sight Distance
Sight distance is the length of 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 a near the design speed to stop before reaching a stationary object in its path. Although greater lengths of visible roadway are desirable, the sight distance at every point along a roadway should be at least that needed for a below-average driver or vehicle to stop.
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.
In computing and measuring stopping sight distances, the height of the driver’s eye is estimated to be 3.5 ft [1080mm] and the height of the object to be seen by the driver is 2.0 ft [600 mm], equivalent to the taillight height of the passenger car.
The calculated and design stopping sight distances are shown in Table 2-1.
The values given in Table 2-1 represent stopping sight distances on level terrain. As a general rule, the sight distance available on downgrades is larger than on upgrades, more or less automatically providing the necessary corrections for grade. Therefore, corrections for grade are usually unnecessary. An example where correction for grade might come into play for stopping sight distance would be a divided roadway with independent design profiles in extreme rolling or mountainous terrain. A Policy on Geometric Design for Highways and Streets, AASHTO, provides additional information and suggested values for grade corrections in these rare circumstances
|
(US Customary) |
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|---|---|---|---|---|
|
|
|
|
Stopping sight distance |
|
|
Design Speed (mph) |
Brake reaction distance (ft) |
Braking distance on level (ft) |
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 |
|
(Metric) |
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|
|
|
|
Stopping sight distance |
|
|
Design Speed (km/h) |
Brake reaction distance (m) |
Braking distance on level (m) |
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 |
|
Note: brake reaction distance predicated on a time of 2.5s; deceleration rate 11.2 ft/sec² [3.4 m/sec²] |
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Anchor: #CHDGIJJG
Decision Sight Distance
Decision sight distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source, recognize the source, select an appropriate speed and path, and initiate and complete the required maneuver safely and efficiently. Because decision sight distance gives drivers additional margin for error and affords them sufficient length to maneuver their vehicles at the same or reduced speed rather than to just stop, its values are substantially greater than stopping sight distance. Table 2-2 shows recommended decision sight distance values for various avoidance maneuvers.
|
(US Customary) |
(Metric) |
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|---|---|---|---|---|---|---|---|---|---|---|---|
|
Decision sight distance (ft) Avoidance maneuver |
Decision sight distance (m) Avoidance maneuver |
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|
Design speed (mph) |
A |
B |
C |
D |
E |
Design speed (km/h) |
A |
B |
C |
D |
E |
|
30 |
220 |
490 |
450 |
535 |
620 |
50 |
70 |
155 |
145 |
170 |
195 |
|
35 |
275 |
590 |
525 |
625 |
720 |
60 |
95 |
195 |
170 |
205 |
235 |
|
40 |
330 |
690 |
600 |
715 |
825 |
70 |
115 |
235 |
200 |
235 |
275 |
|
45 |
395 |
800 |
675 |
800 |
930 |
80 |
140 |
280 |
230 |
270 |
315 |
|
50 |
465 |
910 |
750 |
890 |
1030 |
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