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From page 28... ... 28 This chapter presents a reliability analysis model for horizontal curves with limited SSD due to sight obstructions on the inside of the curve. Any horizontal curve is likely to experience more crashes than a tangent roadway section simply because of the presence of the curve (AASHTO 2010; AASHTO 2014) Read the entire page →
From page 29... ... Reliability Analysis Model for Horizontal Curves with Limited SSD 29 a more realistic assumption for the height of the object to be seen would be 3.5 ft, which is used as the assumed object height in passing sight distance design. In addition, the alternative method assumes that the lateral position of the driver's eye within a given travel lane on a horizontal curve is at 75 percent of the distance from the inside to the outside edge of the travel lane on a curve to the right and at 25 percent of the distance from the inside to the outside edge of the travel lane on a curve to the left. Read the entire page →
From page 30... ... 30 Design Guidelines for Horizontal Sightline Offsets 5.2 Output Data from the Reliability Analysis Model The outputs provided by the reliability model include: • Minimum ASSD at any point in each travel lane for the primary direction of travel; • Length of the sight-restricted area; • A color-coded indication whether the minimum ASSD for each travel lane is less than or greater than or equal to the AASHTO value of DSSD for the applicable design or operating speed; • An estimate of the total number of vehicles per year potentially affected by any sight distance restriction that is present, for each travel lane and for all lanes combined; • Total number of vehicles passing through the horizontal curve site in a year, for comparison to the previous measure; and • An estimate of the percentage of the total number of vehicles per year potentially affected by any sight distance restriction that is present, for each travel lane and for all lanes combined. Figure 13 illustrates the display of these results in the output spreadsheet tool. Read the entire page →
From page 31... ... Reliability Analysis Model for Horizontal Curves with Limited SSD 31 In addition, the model output includes the values of ASSD along the road from Station PC – S to PC + S, so that the point of minimum ASSD can be located and so that the designer can assess the distance over which the ASSD remains at a low level. Figure 14 shows an example of a sight distance profile plotted from the ASSD data in the model output. Read the entire page →
From page 32... ... 32 Design Guidelines for Horizontal Sightline Offsets The reliability model can provide output for two different sets of assumptions concerning sight distance measurement. The first option uses the sight distance measurement assumptions exactly as presented in the AASHTO Green Book (AASHTO 2011) Read the entire page →
From page 33... ... Reliability Analysis Model for Horizontal Curves with Limited SSD 33 distance restriction. The reliability analysis model estimates the number and percentage of vehicles per year potentially affected by any sight distance restriction present. Read the entire page →
From page 34... ... 34 Design Guidelines for Horizontal Sightline Offsets (a) Read the entire page →
From page 35... ... Reliability Analysis Model for Horizontal Curves with Limited SSD 35 curve to a point where ASSD is no longer limited by the sight obstruction. Rather, ASSD as a function of the driver's position on the roadway must be analyzed with a set of equations for a specific scenario applicable to that position on the roadway. Read the entire page →
From page 36... ... 36 Design Guidelines for Horizontal Sightline Offsets vehicle with the ASSD less than DSSD so that their drivers could not see the queue with the full DSSD available. If a stopped vehicle should be present in Segment 2, then the first two vehicles traveling toward the stopped vehicle will be in a situation with ASSD less than DSSD. Read the entire page →
From page 37... ... Reliability Analysis Model for Horizontal Curves with Limited SSD 37 The furthest downstream a stopped vehicle can queue back into the sight-restricted area during hour i is dependent on the flow rate, qiy, in lane y during hour i. Let this distance be expressed by xiy, the number of 25-ft segments downstream of the sight-limited area in lane y in which a crash could occur that would produce a queue long enough to enter the sight-limited area in hour i, determined as: = − (15) Read the entire page →
From page 38... ... 38 Design Guidelines for Horizontal Sightline Offsets The estimated number of vehicles potentially affected when a stopped vehicle is present during hour i in lane y, Viy, is the sum of Equations (11) Read the entire page →
From page 39... ... Reliability Analysis Model for Horizontal Curves with Limited SSD 39 shifted, however, based on the capacity of the facility. The probability of a breakdown in flow resulting in a congestion-related queue during any particular hour with a specified flow rate is computed as: ∫= s π ( ) Read the entire page →
From page 40... ... 40 Design Guidelines for Horizontal Sightline Offsets vehicles per year that will potentially encounter a stopped vehicle or a queue of stopped vehicles, due to a crash or due to flow in excess of capacity, in the sight-restricted area. Approaching vehicles are treated as potentially affected by the sight distance limitation only if the crashinvolved vehicle or the stopped vehicle at the rear of the queue is not visible to the approaching driver over a distance greater than or equal to the applicable value of DSSD. Read the entire page →
From page 41... ... Rural two-lane highway curve to the right, 250-ft radius, 0.20-mi curve length, 60-mph design speed AADT per lane (veh/day) Offset of obstruction from inside edge of traveled way (ft) Read the entire page →
From page 42... ... Rural four-lane freeway curve to the right, 1000-ft radius, 0.20-mi curve length, 75-mph design speed AADT right lane (veh/day) Offset of obstruction from inside edge of traveled way (ft) Read the entire page →
From page 43... ... Reliability Analysis Model for Horizontal Curves with Limited SSD 43 0.011 percent of the total yearly flow. These results indicate that sight-distance-related crashes are highly unlikely on rural two-lane highways, as only a small fraction of the potentially affected vehicles would likely become involved in a crash. Read the entire page →

From page 28...

... 28 This chapter presents a reliability analysis model for horizontal curves with limited SSD due to sight obstructions on the inside of the curve. Any horizontal curve is likely to experience more crashes than a tangent roadway section simply because of the presence of the curve (AASHTO 2010; AASHTO 2014)