4. (1 point) Calculate the length of unflared guardrail required to protect traf

Question

4. (1 point) Calculate the length of unflared guardrail required to protect traffic traveling either direction from striking a 5 ft x 5 ft traffic counting equipment box that is located starting 15 feet off the shoulder. The road has a 70 mph design speed, 12-foot lanes, 8-foot shoulders, and a 4:1 foreslope. The roadway serves an ADT of 7,500 vehicles per day. The guardrail offset from the shoulder is 2 feet. Provide the length of each section of guardrail as well as the total length of guardrail needed. (A drawing to start out is suggested.) (Hints: LA is measured from the edge of the relevant traveled way, which will be different for directional vs. opposing traffic, and the LR runs to the perpendicular edge of the obstacle plus designers are unlikely to leave a gap.)

Explanation / Answer

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Given

we should Calculate the length of unflared guardrail required to protect traffic traveling either direction from striking a5 ftx 5 ft traffic counting equipment box that is located starting 15 feet off the shoulder. The road has a 70 mph design speed, 12-foot lanes, 8-foot shoulders, and a 4:1 foreslope. The roadway serves an ADT of 7,500 vehicles per day. The guardrail offset from the shoulder is 2 feet. Provide the length of each section of guardrail as well as the total length of guardrail needed. (A drawing to start out is suggested.) (Hints: LA is measured from the edge of the relevant traveled way, which will be different for directional vs. opposing traffic, and the LR runs to the perpendicular edge of the obstacle plus designers are unlikely to leave a gap).

For AADT > 750, the following formulas can be used to determine the guard rain length.

Lu=250-250/Dwx Gw
= 250—250/12x8
= 83.33ft
Ld =250-250/Dd*Gd
= 250—250/18*2
= 222.22ft

Where  
Ld = Length of guardrail needed down storm of the area of concern.
Lu = Length of theiguard rail needed, upstream of the concern.

Total length of guard rail = Lu+ Ld
= 8333+22222  
= 305.55 ft
For AADT >750, the following formulas can be used to determine the guard rail length.
Lu = 250—250/Du*Gu
= 250—250/2 *8
=8333ft
Ld= 250—250/Dd*Gd
= 222.22ft
Total length of guard rail = Lu + Ld
= 83.33+22222
=305.55ft

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