All-In-One Example 410.1

[John QPE]

Without posting the entire problem, this is a basic intersection sight distance problem. Case 'A' intersection with no control, building on the corner .... solve for db and pull that distance off the Green Book chart to determine the posted speed limit of the intersecting roadway.

The solution for db is 87.4 ft, which agrees with the solution in the book.

I feel the posted speed limit should be 15mph, not 20 mph ...... as 15 mph give you the required 70-foot approach leg, whereas 20 mph requires 90 feet.

What are your thoughts??

 

[ptatohed]

Without posting the entire problem, this is a basic intersection sight distance problem. Case 'A' intersection with no control, building on the corner .... solve for db and pull that distance off the Green Book chart to determine the posted speed limit of the intersecting roadway.

The solution for db is 87.4 ft, which agrees with the solution in the book.

I feel the posted speed limit should be 15mph, not 20 mph ...... as 15 mph give you the required 70-foot approach leg, whereas 20 mph requires 90 feet.

What are your thoughts??

I see what you mean but I think you may be overthinking it. You're thinking, if we select 20mph, a 90-foot sight triangle leg is required, but the braking distance of the approaching car is 87.4 feet, so he doesn't have the required 90 feet. I don't think these sight triangles are that particular. I think you can simply use GDHS Table 9-3 or, better yet, Figure 9-16 and you'll see that for an 87.4 ft leg, the closest corresponding speed is 20mph.

 

[John QPE]

I see what you're saying. I did use Table 9-3 for this problem, and I think I'd still stick with 15mph (I hope I don't need to make this decision on the test) .... the text above the table states that with an assumed design speed of [X] need a clear sight triangle with legs extending "at least" .......

According to this, you NEED 90 feet, we only have 87.4 feet.

Let's just hope the solution choices are a little more obvious