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Length of Need
Length of need (LON) represents the barrier length recommended to adequately shield a roadside obstacle from an errant vehicle that departs a roadway. Historically, KYTC and other agencies viewed runout length (L_{R}) values published in the Roadside Design Guide (RDG) as excessive (AASHTO 2011). In response, agencies developed alternative methods to determine LON.
This article reviews KYTC’s method for calculating LON, defines variables typically used to calculate LON (Tables 1 and 2), provides worked examples that show how to calculate LON for adjacent and opposing traffic using KYTC’s method, and demonstrates a method to calculate barrier bid quantities. Figures 1 and 2 visualize the variables needed to calculate LON and their relationship to the roadway for adjacent and opposing traffic, respectively.
If a barrier is recommended, refer to the Highway Knowledge Portal Article Clear Zone Concept (coming soon) and Roadside Barriers – Guardrail article, KYTC Highway Design Manual (HD800), KYTC Standard Drawings, and the RDG for information on lateral offsets, barrier deflection, terrain effects, flare rates, and length of need (LON).
Table 1 Information Used to Determine Length of Need  

Variable  Description 
LON (X) 
X_{Back } refers to LON calculated for opposing traffic 
LON Point 

Y 

L_{R } 
L _{ RBack } refers to the runout length calculated for opposing traffic direction. 
L_{A } 

L_{C } 

L_{S } 

L_{1 } 

L_{2 } 

L_{3 } 

a:b 

θ 

Table 2 The Roadside Design Guide's Suggested ShyLine Offset Values  

Design Speed (mph)  ShyLine Offset (L_{S}) 
80  12 
75  10 
70  9 
60  8 
55  7 
50  6.5 
45  6 
40  5 
30  4 
Table 3 The Roadside Design Guide's Suggested ClearZone Distances (In Feet) From Edge of the Through Traveled Lane  

Design Speed (mph)  Design ADT  Foreslopes  Backslopes  
1V:6H or flatter  1V:5H to 1V:4H  1V:3H  1V:3H  1V:5H to 1V:4H  1V:6H or flatter  
< 40 
Under 750^{c} 750  1500 1500  6000 Over 6000 
710 1012 1214 1416 
710 1012 1416 1618 
See Note b 
710 1012 1214 1416 
710 1012 1214 1416 
710 1012 1214 1416 
45  50 
Under 750^{c} 750  1500 1500  6000 Over 6000 
1012 1416 1618 2022 
1214 1620 2026 2428 
See Note b 
810 1012 1214 1416 
810 1214 1416 1820 
1012 1416 1618 2022 
55 
Under 750^{c} 750  1500 1500  6000 Over 6000 
1214 1618 2022 2224 
1418 2024 2430 2632^{a} 
See Note b 
810 1012 1416 1618 
1012 1416 1618 2022 
1012 1618 2022 2224 
60 
Under 750^{c} 750  1500 1500  6000 Over 6000 
1618 2024 2630 3032^{a} 
2024 2632^{a} 3240^{a} 3644^{a} 
See Note b 
1012 1214 1418 2022 
1214 1618 1822 2426 
1416 2022 2426 2628 
65  70^{d} 
Under 750^{c} 750  1500 1500  6000 Over 6000 
1820 2426 2832^{a} 3034^{a} 
2026 2836^{a} 3442^{a} 3846^{a} 
See Note b 
1012 1216 1620 2224 
1416 1820 2224 2630 
1416 2022 2628 2830 
Source: RDG July 2015 Errata (Table 31)
Notes
a. When a sitespecific investigation indicates a high probability of continuing crashes or when such occurrences are indicated by crash history, the designer may provide clearzone distances greater than the clear zone shown in Table 3. Clear zones may be limited to 30 feet for practicality and to provide a consistent roadway template if previous experience with similar projects or designs indicates satisfactory performance.
b. Because recovery is less likely on the unshielded, traversable 1V:3H fill slopes, fixed objects should not be present in the vicinity of the toe of these slopes. Recovery of highspeed vehicles that encroach beyond the edge of the shoulder may be expected to occur beyond the toe of slope. Determination of the width of the recovery area at the toe of slope should consider right of way availability, environmental concerns, economic factors, safety needs, and crash histories. Also, the distance between the edge of the through traveled lane and the beginning of the 1V:3H slope should influence the recovery area provided at the toe of slope. While the application may be limited by several factors, the foreslope parameters that may enter into determining a maximum desirable recovery area are illustrated in Figure 41. A 10foot recovery area at the toe of slope should be provided for all traversable, nonrecoverable fill slopes.
c. For roadways with low volumes, it may not be practical to apply even the minimum values found in Table 3. Refer to RDG Chapter 12 for additional considerations for lowvolume roadways and RDG Chapter 10 for additional guidance for urban applications.
d. When design speeds are greater than the values provided, the designer may provide clearzone distances greater than those shown in Table 3.
The RDG provides adjustment factors for horizontal curvature that may be applied to distances in Table 3. Table 4 reproduces these adjustment factors.
Table 4 The Roadside Design Guide's Horizontal Curve Adjustment Factor for Clear Zones  

Radius (ft)  Design Speed (mph)  
40  45  50  55  65  70  
2950  1.1  1.1  1.1  1.2  1.2  1.2 
2300  1.1  1.1  1.2  1.2  1.2  1.3 
1970  1.1  1.2  1.2  1.2  1.3  1.4 
1640  1.1  1.2  1.2  1.3  1.3  1.4 
1475  1.2  1.2  1.3  1.3  1.4  1.5 
1315  1.2  1.2  1.3  1.3  1.4   
1150  1.2  1.2  1.3  1.4  1.5   
985  1.2  1.3  1.4  1.5  1.5   
820  1.3  1.3  1.4  1.5     
660  1.3  1.4  1.5       
495  1.4  1.5         
330  1.5           
Source: RDG Table 32
CZ_{C}=(L_{C})*(K_{CZ})
where:
CZ_{C }= Clear zone on outside of curvature, feet
L_{C} = Clear zone distance, feet (see Table 21)
K_{CZ }= Curve correction factor
Note: The clearzone correction factor is applied to the outside of curves only. Corrections are typically made only to curves less than 2,950ft radius.
When determining where to place a roadside barrier, KYTC’s method uses a fixed vehicle departure angle of 15 degrees from the edge of the travelled way to the obstacle. This method adopts all of the variable definitions and notations presented in Table 1. Figure 4 illustrates the setup and Figure 5 is for reference, to use when calculating LON.
Problem 1a Calculating LON
Use KYTC’s method and one of the trig functions to calculate LON(X), designated X_{Front}, for a facility that has an ADT of 6,200 veh/day, a design speed of 70 mph, and embankment slopes on the right side of the highway of 1V:6H (i.e., the side next to the adjacent traffic lane). As per KYTC’s method, θ = 15°. Assume the lateral extent of the obstacle (L_{A}) = 30 ft and the distance from the edge of the traveled way to the guardrail face (L_{2}) = 10 ft. Figure 6 illustrates the setup for this problem.
StepbyStep Instructions
 Use Table 3 to determine the appropriate clear zone width (L_{C}) based on traffic volume (6,200 veh/day), design speed (70 mph), and slope (1V:6H):
L_{C} = 34 ft, L_{S}_{ }= 9 ft (from Table 2), and L_{2}= 10 ft
Since L_{S} < L_{2}, the barrier was placed outside the shyline and should not be perceived as an obstacle to the driver. Refer to the definition in Table 1 and the RDG for more information concerning the shyline.
Compare L_{A} to L_{C }to determine which value should be used to calculate X_{Front} — L_{A} = 30 ft or L_{C} = 34 ft. In this example only one obstacle is within the clear zone (L_{C}), and a barrier would need to shield this obstacle at the lateral distance L_{A}._{ }Therefore, L_{A} will be used in the calculation.
2. Select the appropriate trig function. Since information is given on L_{A} and L_{2} (which is opposite of θ), use the tan function:
3. Substitute known information into the equation above. θ = 15°. Calculate the numerator by subtracting L_{2} from L_{A} (or, 30 – 10):
4. Rearrange the equation to solve for X_{Front}:
5. Simplify the equation and calculate X_{Front}:
Solution:
Problem 1b Calculating Runout Length
When performing the field check method (see Section 5, Conducting a LON Field Check), calculating runout length (L_{R}) is necessary. Designers and field personnel can use this value to verify if the length of roadside shielded by a barrier is sufficient. Using the same parameters as Problem 1a, calculate L_{R} using KYTC’s method. As per KYTC’s method, θ = 15°. The lateral extent of the area of concern (L_{A}) = 30 ft.
StepbyStep Instructions
1. Select the appropriate trig function. Since information is given on L_{A} (which is opposite of θ), use the tan function:
2. Substitute known information into the equation above. θ = 15°. The numerator is L_{A} (30):
3. Rearrange the equation to solve for L_{R}:
4. Simplify the equation and calculate L_{R}:
Solution:
Problem 2a Protecting Opposing Traffic
On twoway roads, opposing traffic must also be shielded from obstacles on the opposite side of the road (i.e., the trailing end). While the same method is used to determine LON(X), all lateral dimensions are measured from the opposing traffic’s edge of traveled way which, for the driver’s side, is the highway centerline.
Use KYTC’s method and one of the trig functions to calculate LON, designated X_{Back }, for a facility that has an ADT of 6,200 veh/day, a design speed of 70 mph, and embankment slopes on the right side of the highway of 1V:6H. As per KYTC’s method, θ = 15°. Assume the lateral extent of the area of concern (L_{A}) = 42 ft and the distance from the edge of the traveled way to the guardrail face (L_{2}) = 22 ft. Compared to Problems 1a and 1b, values for L_{A} and L_{2} are 12 ft greater. This is because these values are measured from the centerline and the lanes have a width of 12 ft. Figure 7 illustrates the setup for this problem.
Figure 7 Setup for Example Problems 2a and 2b
StepbyStep Instructions
 Use Table 1 to determine the appropriate clear zone width (L_{C}) based on traffic volume (6,200 veh/day), design speed (70 mph), and slope (1V:6H):L_{C} = 34 ft
2. Compare L_{A} to L_{C }to determine which value should be used to calculate X_{Back} — L_{A} = 42 ft or L_{C} = 34 ft. If L_{A} lies beyond the appropriate clear zone (L_{C}), the designer may choose to shield only the portion within the clear zone. Because L_{A} > L_{C}, use L_{C} in the calculation.
3. Select the appropriate trig function. Based on Figure 6, calculate the length of the triangle leg adjacent to θ. Since information is given on L_{C} (which is opposite of θ), use the tan function:
4. Substitute known information into the equation above. θ = 15°. Calculate the numerator by subtracting L_{2} from L_{C} (or, 34 – 22):
5. Rearrange the equation to solve for X_{Back}:
6. Simplify the equation and calculate X_{Back}:
Solution:
Problem 2b Calculating Runout Length
When performing the field check method (see Section 5, Conducting a LON Field Check), a designer must calculate runout length (L_{R}). Designers can use this value to verify that enough of the roadside is shielded by a barrier. Using the same parameters as Problem 2a, calculate the runout for the opposing traffic direction (LR_{Back} ) using KYTC’s method. As per KYTC’s method, θ = 15°. The clear zone (L_{C}) = 34 ft.
StepbyStep Instructions
1. Select the appropriate trig function. Since information is given on L_{C} (which is opposite of θ), use the tan function:
2. Substitute known information into the equation above. θ = 15°. The numerator is L_{C} (34). Solve for LR_{Back}, which is the denominator:
3. Rearrange the equation to solve for LR_{Back} :
4. Simplify the equation and calculate LR_{Back} :
Solution:
Once a designer calculates LON for the front and back sides of an obstacle, they need to calculate the total length of guardrail. A construction note specifies this value on the plans. Guardrail length is included in the total guardrail bid item quantity.
Guardrail End Treatments (which are bid as EACH) usually include lengths of guardrail capable of redirecting an errant vehicle. This guardrail is included in and paid for by the end treatment bid item. As such, this length of guardrail is not included in the guardrail bid item quantity.
For example, KYTC’s Standard Drawing RBR020 (Figure 4) shows a Guardrail End Treatment Type 1 has a pay limit of 50 feet and Length of Need Point at the third post. With a post spacing of 6 ft, 3 in, the total distance between the first and third posts is 12.5 ft. For a Guardrail End Treatment Type 1, the total length of redirective guardrail is 37.5 ft (arrived at by calculating 50 ft – 12.5 ft). The 37.5 ft is included in the length of the guardrail system needed to shield the obstacle, but it is not included in the length of guardrail bid quantity. See KYTC Standard Drawings for Barriers for additional information.
Problem 3 Calculating Guardrail Bid Item Quantities
Determine the guardrail bid item quantity for the example given in Figure 8. Assume that Guardrail End Treatment Type 1 will be used. Because guardrail sections are manufactured in standard lengths of 12.5 ft, when calculating quantities round up the length to the nearest multiple of 12.5 ft.
StepbyStep Instructions
 Calculate the bid item quantity for Length_{Back} (which captures LON for opposing traffic)
Guardrail Length_{Back} = LON_{Back} – Length of End Treatment Redirective Section
2. Substitute known information from Figure 8 into the equation above.
Guardrail Length_{Back} = 50 – 37.5 = 12.5 ft
3. Calculate the bid item quantity for Length_{Front} (which captures LON for adjacent traffic).
Guardrail Length_{Front} = LON_{Front} – Length End Treatment Redirective Section
4. Substitute known information from Figure 8 into the equation above.
Guardrail Length_{Front} = 112.50 – 37.5 = 75 ft
5. Sum all values. Recall that the guardrail length adjacent to obstacle area = 100 ft from Figure 8. Because 12.5 ft (Step 2) and 75 ft (Step 4) are multiples of 12.5 ft, these values do not need to be rounded up.
Total Length of Guardrail = 12.5 + 100 + 75 = 187.5 ft
For this example, a typical construction note shown on the plan sheet would read: From Lt. Sta. 99+37.5 to Lt. Sta. 102+25 Construct 187.5 ft. Steel “W” Beam Guardrail (Single Face) 2 End Treatment Type 1’s.
Construction Inspectors or Maintenance Personnel should perform a LON field check to verify the barrier provided is sufficiently long. The check may be performed during barrier installation while proper traffic control is in place. If done when traffic is using adjacent lanes, take proper safety precautions (e.g., wearing high visibility clothing). Each step of the process is described and illustrated below.
3. From that point, turn around and look at the back of the obstacle.
If the obstacle has no clear side, (e.g., a river or embankment), this distance may be limited to the design clear zone. If the proposed (or actual) guardrail installation crosses (or is close to) that line of sight, the area is adequately covered. If not, consider extending the barrier to shield the obstacle properly.
6. From that point, turn around and look at the back of the obstacle.
If the obstacle has no clear far side (e.g., a river or embankment), this distance may be limited to the design clear zone — as measured from the centerline. If the proposed (or actual) guardrail installation crosses (or is close to) that line of sight, the area is adequately covered. If not, consider extending the barrier to shield the obstacle properly.
Pay special attention to all obstacles in the immediate vicinity that need to be shielded. Less apparent obstacles (e.g., steep transverse embankment within the clear zone) located near an obvious obstacle (i.e., a bridge pier) should be considered when determining adequate barrier length.
KYTC guidance advises that guardrail extend at least 200 ft upstream of a fixed object. This includes the end treatment. However, the length may be reduced if field conditions warrant.
The 200 ft threshold is used to establish the minimum effective length of Strong Post Wbeam guardrail (sometimes referred to as guardrail ribbon strength). Take this minimum length under consideration when calculating LON upstream and downstream of a fixed object.
Based on MASH Test Level 3 crash testing, the recommended minimum length for the standard Midwest Guardrail System (MGS) is 75 ft. Variability in roadway geometrics and/or the number and arrangement of roadside obstacles may produce LON calculations shorter than KYTC’s recommended minimum length. When the LON calculations are shorter than 200 ft, discuss sitespecific considerations with the project manager and project team when making decisions about LON.
KYTC
Highway Knowledge Portal Articles
Roadside Barriers – Guardrail: https://kp.uky.edu/knowledgeportal/articles/guardrailversion23/
Clear Zone Concept (coming soon)
AASHTO Roadside Design Guide (4th Edition 2011)
North Dakota DOT
Length of Need Field Check Section