Freeway segment (downgrade) composite method

[MaryJ]

In Highway Capacity Manual example problem #4, gives an example of a freeway segment with grade in segment one of 3% and length of 3000 ft. Segment two of the said freeway has grade of 5% and length of 2600 ft. They gives other parameters then asked to find the LOS for the upgrade and downgrade direction. The solution for the upgrade section is given which I totally understand, but for the life of me, I cannot seem to come up with the same answers for the downgrade direction as given in the solution. HCM states that the answer should be -1% for the downgrade grade using the composite method analysis. Can someone help me out with this problem please.

This is my approach and I always get stuck. In solving for the downgrade, I reversed the order of the grades, thus calling the original segment two going upgrade----now is segment one going downgrade and original segment one going upgrade ---is now segment two going downgrade, since approaching from the opposite direction.

I then enter 2600 ft at the bottom of the graph (Exhibit A23-2) and draw a vertical line to the 3% performance curve, then draw a second horizontal line going to the left of the y-axis of the graph and see that it intersects with 30mph. At this point there is no way to connect from the 3% to a 5% performance curve to continue to solve the problem, as anyone can see. How is this done? Anyone?

 

[Ambrug20]

In Highway Capacity Manual example problem #4, gives an example of a freeway segment with grade in segment one of 3% and length of 3000 ft. Segment two of the said freeway has grade of 5% and length of 2600 ft. They gives other parameters then asked to find the LOS for the upgrade and downgrade direction. The solution for the upgrade section is given which I totally understand, but for the life of me, I cannot seem to come up with the same answers for the downgrade direction as given in the solution. HCM states that the answer should be -1% for the downgrade grade using the composite method analysis. Can someone help me out with this problem please.
This is my approach and I always get stuck. In solving for the downgrade, I reversed the order of the grades, thus calling the original segment two going upgrade----now is segment one going downgrade and original segment one going upgrade ---is now segment two going downgrade, since approaching from the opposite direction.

I then enter 2600 ft at the bottom of the graph (Exhibit A23-2) and draw a vertical line to the 3% performance curve, then draw a second horizontal line going to the left of the y-axis of the graph and see that it intersects with 30mph. At this point there is no way to connect from the 3% to a 5% performance curve to continue to solve the problem, as anyone can see. How is this done? Anyone?

Mary,

may be I didn't understand your question well, but in this problem you were asked to find LOS for this road. Why do you need to mach grades, when they are given to you. Make it simple. Find LOS for these grades. The procedure is very well explained in steps below the problem. All these graphs your are trying to use, almost impossible to use for these type of problems, because they all "aproximate". Don't make it harder what its already hard enough. :tardbang: