Sight Distance. The availability of sufficient sight distance for the driver to see ahead is critical to the design of a safe highway.

Transcription

1 Sigt Distance Te availability of sufficient sigt distance for te driver to see aead is critical to te design of a safe igway. Wat is sigt distance? Sigt distance is te lengt of igway visible to a driver. A safe sigt distance is te distance needed by a driver on an arterial, or a driver exiting a driveway or street, to verify tat te road is clear and avoid conflicts wit oter veicles. Sigt lines must be kept free of objects wic migt interfere wit te ability of drivers to see oter veicles. Features suc as ills, curves in te road, vegetation, oter landscaping, signs, and buildings can reduce sigt distance. Wat about in rural areas? Sigt distance at driveways and/or farm field entrances is especially critical in rural areas because travel speeds are ig and curves or illy terrain may restrict sigt lines. Higer veicle speeds mean tat driver reaction and pysical stopping distances are longer. Te most common types of sigt distance in roadway design are stopping sigt distance, and passing sigt distance. 1. Stopping sigt distance: Stopping sigt distance is defined as te distance needed for drivers to see an object on te roadway aead and bring teir veicles to safe stop before colliding wit te object. Te distances are derived for various design speeds based on assumptions for driver reaction time, te braking ability of most veicles under wet pavement conditions, and te friction provided by most pavement surfaces, assuming good tires. Stopping sigt distance is influenced by bot vertical and orizontal alignment. For vertical sigt distance, tis includes stopping sigt distance or passing sigt distance at crest vertical curves and eadligt sigt distance at sag vertical curves. Sag vertical curves provide greater stopping sigt distance during dayligt conditions, but very sort sag vertical curves will limit te effective distance of te veicle s eadligts at nigt. If ligting is provided at sag vertical curves, a design to te driver comfort criteria may be adequate. Te lengt of sag vertical curves to satisfy te comfort criteria over te typical design speed

2 range results in minimum curve lengts of about alf tose based on eadligt criteria. Stopping sigt distance is te sum of te brake reaction distance and te braking distance. Actual stopping distances are also affected by road conditions, te mass of te car, te incline of te road, and numerous oter factors. V : speed of veicle (km/r). f : Coefficient of friction. Ss = V.5V + s g f 100 S : Te percent of grade divided by 100 (+ve upill and ve downill) Note: Te stopping distances needed on upgrades are sorter tan on level roadways; tose on downgrades are longer. Ss = 0.78V t V a Table 1 : SSD on level roadways. Te stopping sigt distances for various grades are sown in table. Tese adjusted sigt distance values are computed for wet-pavement conditions using te same design speeds and brake reaction times used for level roadways in Table1.

3 Table : SSD on grade Sigt distance wit vertical curves Te longest distance troug wic a driver wit is or er eye eigt of 1.05 m can see an object wit a 0.15 m eigt. L min : Minimum lengt of vertical curve so tat te required sigt distance is acieved.

4 if S L : L if S L : L min min 00 NS 1 00 S 1 N, N g g 1 g: Slope of tangent of vertical curve, expressed as percentage. Sigt Distance wit curves Required stopping sigt distance (SSD), used to calculate te minimum rate of vertical curvature or te minimum lengt of a crest vertical curve (CVC), is usually calculated on te assumption tat te grade of te braking section is zero (G = 0). Tis assumption is not te worst case and may lead to te selection of a CVC on wic tere is a segment wit SSD restriction. On eac CVC tere are braking sections on one or bot directions of travel, on wic average grade is negative (G < 0). Nowadays, a metod for calculating required SSD, partly or wolly on a CVC, is developed, using te average grade over te braking distance. In addition, a computer program is compiled for calculating te value of te required SSD for all driver positions before or on a CVC.. Passing Sigt Distance: Most roads and numerous streets are considered to qualify as two-lane, twoway igways on wic veicles frequently overtake slower moving veicles, te passing of wic must be accomplised on lanes regularly used by opposing traffic. If passing is to be accomplised wit safety, te driver sould be able to see a sufficient distance aead, clear of traffic, to complete te passing maneuver witout cutting off te passed veicle in advance of meeting an opposing veicle appearing during te maneuver. Wen required, a driver can return to te rigt lane witout passing if e or se sees opposing traffic is too close wen te maneuver is only partially completed. Wen computing minimum passing sigt distances on two-lane igways for design use, certain assumptions for traffic beavior are necessary, some of wic offer a wide coice. Te assumed control for driver beavior sould be tat practiced by a ig percentage of drivers, rater tan te average driver. Suc assumptions follow:

5 1. Te overtaken veicle travels at uniform speed.. Te passing veicle as reduced speed and trails te overtaken veicle as it enters a passing section. 3. Wen te passing section is reaced, te driver requires a sort period of time to perceive te clear passing section and to react to start is or er maneuver. 4. Passing is accomplised under wat may be termed a delayed start and urried return in te face of opposing traffic. Te passing veicle accelerates during te maneuver, and its average speed during te occupancy of te left lane is 15 km/ iger tan tat of te overtaken veicle. 5. Wen te passing veicle returns to its lane, tere is a suitable clearance lengt between it and an oncoming veicle in te oter lane. Te minimum passing sigt distance for two-lane igways is determined as te sum of te four distances: d1 - Distance traversed during perception and reaction time and during te initial acceleration to te point of encroacment on te left lane. d - Distance traveled wile te passing veicle occupies te left lane. d3 - Distance between te passing veicle at te end of its maneuver and te opposing veicle. d4 - Distance traversed by te opposing veicle for two-tirds of te time te passing veicle occupies te left lane, or /3 of d above.

6 Table 3 : PSD values for different speeds. PSD > SSD For Divided roads, Te passing sigt distance is not available. For roads tat ave special lane for overtaking, Te passing sigt distance is not available. Wen passing sigt distance is not available it equals SSD.

7 Safety Effects: Te adopted criteria for stopping sigt distance apply to te entire lengt of a igway. Clearly toug, te relative risk of limited sigt distance can vary significantly, based on te circumstances. A simple model for evaluating locations wit limited sigt distance involves te following questions: Wat roadway or oter conditions or features are witin te segment wit limited sigt distance? How significant is te deficiency in sigt distance (as measured by lengt of igway as well as amount of deficiency relative to tat required per adopted criteria)? Wat is te traffic volume troug te location wit limited sigt distance? For example, te risk associated wit a crest vertical curve wit nonstandard sigt distance is greater at a location wit intersections or driveways or oter roadway features witin te area of te sigt restriction compared wit a similar location wit no suc features. Table 4 below summarizes te relative safety risk of combining various geometric elements and oter roadway features wit non-standard stopping sigt distance. Geometric Condition Tangent orizontal alignment Mild curvature >000 ft (600m) radius Mild downgrade (<3%) Low-volume intersection Intermediate curvature 300 m to 600 m radius Moderate downgrade (3 5%) Hig volume intersection Sarp curvature < 300 m radius Relative Safety Risk Minor Significant Major Steep downgrade (>5%) Narrow bridge Narrow pavement Exit or entrance downstream along freeway Table 4 : Relative Safety Risk of Various Conditions in Combination wit Non-Standard Stopping Sigt Distance.