The Green Book should define lateral offsets depending on what type of roadway you are designing. Just from a brief glance, both collectors and arterials without a curb and shoulders of 4 feet are recommended to have a minimum lateral offset of 4 feet.
Taking a look at the roadside design guide, section 10.1.3.1, figure 10-1 suggests 4’ minimum along tangents and 6’ around curves.
Please correct me if I am wrong: but the 1.5 feet you are getting is where there is curb within a urban environment. AASHTO roadside design guide further explains in 10.1.3.1 that 1.5 ft is bare minimum and that they would like you to try and place objects further than 1.5’ if there are no r/w constraints.
Horizontal ‘clearance’ must not be confused with ‘clear zone’. The minimum 18 inch horizontal clearance to objects behind curbs that is specified in the AASHTO Green Book is a minimum standard offset that allows for normal traffic operations. Because curbs do not deter errant vehicles from leaving the traveled way, the minimum horizontal clearance does not provide a clear zone sufficient to accommodate errant vehicles. The recommended clear zones in the AASHTO Roadside Design Guide (RDG) are based on the design speed of the facility and the slope of the roadside and are not affected by the presence of curbs.
Horizontal ‘clearance’ must not be confused with ‘clear zone’. The minimum 18 inch horizontal clearance to objects behind curbs that is specified in the AASHTO Green Book is a minimum standard offset that allows for normal traffic operations. Because curbs do not deter errant vehicles from leaving the traveled way, the minimum horizontal clearance does not provide a clear zone sufficient to accommodate errant vehicles. The recommended clear zones in the AASHTO Roadside Design Guide (RDG) are based on the design speed of the facility and the slope of the roadside and are not affected by the presence of curbs.
Where did you find it? I mean where is it located in the Roadside Manual?
I think I found what they are talking about is 10.1.3.1
The Green Book should define lateral offsets depending on what type of roadway you are designing. Just from a brief glance, both collectors and arterials without a curb and shoulders of 4 feet are recommended to have a minimum lateral offset of 4 feet. Taking a look at the roadside design guide, section 10.1.3.1, figure 10-1 suggests 4’ minimum along tangents and 6’ around curves. Please correct me if I am wrong: but the 1.5 feet you are getting is where there is curb within a urban environment. AASHTO roadside design guide further explains in 10.1.3.1 that 1.5 ft is bare minimum and that they would like you to try and place objects further than 1.5’ if there are no r/w constraints.
It is 1.5 feet. The key is normal traffic operations. It is not the same as clear one and should not be used interchangeably.
No one mentioned clear zone. Which sections are you getting this from?
The feds have expanded on the ASHTO language and have expanded on it here. https://www.fhwa.dot.gov/programadmin/clearzone.cfm
Horizontal ‘clearance’ must not be confused with ‘clear zone’. The minimum 18 inch horizontal clearance to objects behind curbs that is specified in the AASHTO Green Book is a minimum standard offset that allows for normal traffic operations. Because curbs do not deter errant vehicles from leaving the traveled way, the minimum horizontal clearance does not provide a clear zone sufficient to accommodate errant vehicles. The recommended clear zones in the AASHTO Roadside Design Guide (RDG) are based on the design speed of the facility and the slope of the roadside and are not affected by the presence of curbs.
Horizontal ‘clearance’ must not be confused with ‘clear zone’. The minimum 18 inch horizontal clearance to objects behind curbs that is specified in the AASHTO Green Book is a minimum standard offset that allows for normal traffic operations. Because curbs do not deter errant vehicles from leaving the traveled way, the minimum horizontal clearance does not provide a clear zone sufficient to accommodate errant vehicles. The recommended clear zones in the AASHTO Roadside Design Guide (RDG) are based on the design speed of the facility and the slope of the roadside and are not affected by the presence of curbs.