Question about Intersection Sight Distance (ISD) - GDHS Table footnotes

In the AASHTO GDHS, Chapt 9, Section 9.5 (Intersection Sight Distance) - and specifically for Tables 9-5 and 9-7: How do you interpret this footnote? (please see highlighted portion of the Table 9-7 attachment).

I first interpreted it as add 0.1s for each % grade over 3%. But, I guess it could mean add 0.1s for every % over 0%, right?

So, if you had a combo truck on a +5% (up)grade, would it be tg = 10.5 + .1 + .1 = 10.7s; or 10.5 + .1 + .1 + .1 + .1 + .1 = 11.0s?

How do you guys interpret this footnote?

Thanks.

ptatohed said:

In the AASHTO GDHS, Chapt 9, Section 9.5 (Intersection Sight Distance) - and specifically for Tables 9-5 and 9-7: How do you interpret this footnote? (please see highlighted portion of the Table 9-7 attachment).

I first interpreted it as add 0.1s for each % grade over 3%. But, I guess it could mean add 0.1s for every % over 0%, right?

So, if you had a combo truck on a +5% (up)grade, would it be tg = 10.5 + .1 + .1 = 10.7s; or 10.5 + .1 + .1 + .1 + .1 + .1 = 11.0s?

How do you guys interpret this footnote?

Thanks.

Click to expand...

With this much ambiguity, I think I might need a faster car.

11 seconds is correct.

10.5 + (0.1 secs * 5%) = 11 secs

I have this written out in my EET notes and Green Book. I'm sure it came up.

John QPE said:

11 seconds is correct.

10.5 + (0.1 secs * 5%) = 11 secs

I have this written out in my EET notes and Green Book. I'm sure it came up.

Click to expand...

I knew you'd know the answer JQP, thanks. I don't doubt that you are right, trust me, but I am still having a hard time with this. It seems that if the trigger to adjust the tg isn't until the upgrade exceeds 3%, then it seems (in my view) that 0-3% is considered "flat" and that the tg's given in the tables already include grades up to 3%. And thus (in my view) the adjustment would only be applied to each % over 3. That's how I see it. But, I'm probably wrong. /

I would think of it as a linear graph. As steepness increases, common sense will tell you that the time gap would not increase at the same rate, especially with a combo truck.

Does an increase of only 0.5 seconds for a grade of 8% really seem sufficient as opposed to 10.5 for 0% ?

I thought this went your way as well, I must have been corrected in my class, because as I said, I had this written out in two different books.

With ya on this JQ. The way I always understood it was that the difference between 0% and +3% was negligible. Definitely not a very clear footnote.