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Section 3: Sight Distance

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Overview

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:

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Stopping 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 [1080 mm] 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. AASHTO’s A Policy on Geometric Design for Highways and Streets, provides additional information and suggested values for grade corrections in these rare circumstances.

Anchor: #i1319617Table 2-1: Stopping Sight Distance

 

 

 

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

Note: brake reaction distance predicated on a time of 2.5s; deceleration rate 11.2 ft/sec²



NOTE: Online users can view the metric version of this table in PDF format.

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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.

Anchor: #i1318657Table 2-2: Recommended Decision Sight Distance Values

Decision sight distance (ft) Avoidance maneuver

Design speed (mph)

A

B

C

D

E

30

220

490

450

535

620

35

275

590

525

625

720

40

330

690

600

715

825

45

395

800

675

800

930

50

465

910

750

890

1030

55

535

1030

865

980

1135

60

610

1150

990

1125

1280

65

695

1275

1050

1220

1365

70

780

1410

1105

1275

1445

75

875

1545

1180

1365

1545

80

970

1685

1260

1455

1650

Avoidance Maneuver A: Stop on rural road – t = 3.0s

Avoidance Maneuver B: Stop on urban road – t = 9.1s

Avoidance Maneuver C: Speed/path/direction change on rural road – t varies between 10.2 and 11.2s

Avoidance Maneuver D: Speed/path/direction change on suburban road – t varies between 12.1 and 12.9s

Avoidance Maneuver E: Speed/path/direction change on urban road – t varies between 14.0 and 14.5s



NOTE: Online users can view the metric version of this table in PDF format.

Examples of situations in which decision sight distance is preferred include the following:

  • Interchange and intersection locations where unusual or unexpected maneuvers are required (such as exit ramp gore areas and left-hand exits)
  • Changes in cross section such as toll plazas and lane drops
  • Areas of concentrated demand where there is apt to be “visual noise” whenever sources of information compete, as those from roadway elements, traffic, traffic control devices, and advertising signs

Locations along the roadway where a driver has stopping sight distance but not the extra response time provided by decision sight distance is identified as a reduce decision zone. During the design process, the roadway engineer can avoid the location of intersections within a reduced decision zone either by relocating the intersection or by changing the grades to reduce the size of the reduced design zone.

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Passing Sight Distance

Passing sight distance is applicable only in the design of two-lane roadways (including two-way frontage roads) and therefore is presented in Chapter 3, Section 4 under the discussion on “ Two Lane Rural Highways”, and Chapter 4, Section 6 under the discussion on “ Super 2 Highways”,

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Intersection Sight Distance

The operator of a vehicle approaching an intersection should have an unobstructed view of the entire intersection and an adequate view of the intersecting highway to permit control of the vehicle to avoid a collision. When designing an intersection, the following factors should be taken into consideration:

  • Adequate sight distance should be provided along both highway approaches and across corners.
  • Gradients of intersecting highways should be as flat as practical on sections that are to be used for storage of stopped vehicles.
  • Combination of vertical and horizontal curvature should allow adequate sight distance of the intersection.
  • Traffic lanes should be clearly visible at all times.
  • Lane markings and signs should be clearly visible and understandable from a desired distance.
  • Intersections should be free from the sudden appearance of potential conflicts.
  • Intersections should be evaluated for the effects of barriers, rails, and retaining walls on sight distance.

    For selecting appropriate intersection sight distance, refer to AASHTO’s A Policy on Geometric Design for Highways and Streets. Sight distance criteria are provided for the following types of intersection controls:

  • Intersections with no control
  • Intersections with stop control on the minor road
  • Intersections with yield control on the minor road
  • Intersections with traffic signal control
  • Intersections with all-way stop control
  • Left turns from the major road.
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