# culculating DDHV



## Ambrug20 (Oct 6, 2010)

Maximus808 posted the question in Aug.27, but I missed it. He is the question by Max.

“I'm doing a sample problem that gives me the AADT but no value for D or K. It states that the multilane highway is in a low-density suburban area. The solution assumes values for K=0.093 and D=0.60. Do you know where we can find these default values for K and D if these are not given to us. The problem is to calculate the number of lanes to meet LOS C for a divided multilane highway. Speed is also not given which we assume as 60mph and since it is divided we factor that in for to find the FFS. Can anyone help me out, thanks. “

CERM p. 73-4 states K-factor default is 0.09(urban) &amp; 0.1 (rural) other references HCM 8-11;

CERM p 73-5 (on the top left) states that D (direc. Factor) is 80%(rural), 50% central business, and 60% (0.6) is default.

Max,

can you post the problem and answer, please or pdf at [email protected]thanks


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## maximus808 (Oct 6, 2010)

Tanya, sorry my memory is bad. I don't remember this problem, it might have been from a six minute solutions or the practice problems from lindeburg. I'll try and check. But I believe my concern was in regards to the default values. I will print this post as it will be useful for the default values for K and D. If these are not given, we have to use the defaults based on the area to calculate the DDHV. This along with FFS can determine our calculated flowrate (vp) which then need to compare to the max flowrate at the given speed or FFS at the given LOS. The only unknown at this point should be the number lanes which we must continue to change and to lower the calculated vp until it is below the max vp at the given speed and LOS. The tricky part is the by changing the number of lanes, this should change the FFS as it is there are reduction factors for the number of lanes that need to be subtracted from the BFFS. I'm not certain of the exact problem but that would be my approach. If I am given a design speed I will reduce this speed using the number of lanes and I will use that adjusted speed or FFS to determine the max flowrate for the given LOS. It's almost like hydraulics. If the design flow is greater than your allowable for the pipe you are given, you must increase the size of the pipe. Same goes for traffic, you need in increase the number of lanes to allow less traffic per lane.....damn...that wasn't a good example....but same iterative process. I hope that helps.


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## Ambrug20 (Oct 6, 2010)

maximus808 said:


> Tanya, sorry my memory is bad. I don't remember this problem, it might have been from a six minute solutions or the practice problems from lindeburg. I'll try and check. But I believe my concern was in regards to the default values. I will print this post as it will be useful for the default values for K and D. If these are not given, we have to use the defaults based on the area to calculate the DDHV. This along with FFS can determine our calculated flowrate (vp) which then need to compare to the max flowrate at the given speed or FFS at the given LOS. The only unknown at this point should be the number lanes which we must continue to change and to lower the calculated vp until it is below the max vp at the given speed and LOS. The tricky part is the by changing the number of lanes, this should change the FFS as it is there are reduction factors for the number of lanes that need to be subtracted from the BFFS. I'm not certain of the exact problem but that would be my approach. If I am given a design speed I will reduce this speed using the number of lanes and I will use that adjusted speed or FFS to determine the max flowrate for the given LOS. It's almost like hydraulics. If the design flow is greater than your allowable for the pipe you are given, you must increase the size of the pipe. Same goes for traffic, you need in increase the number of lanes to allow less traffic per lane.....damn...that wasn't a good example....but same iterative process. I hope that helps.


I was thinking this way, but wan't sure if that is the answer. thanks


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## pmblair (Oct 6, 2010)

I didnt think this was on the AM, breadth, is directional volume on breadth?????

Becasue i didnt review it!


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## maximus808 (Oct 6, 2010)

Pmblair, confirmed. DDHV is in traffic capacity which is only in the trans depth.


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