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Merge Analysis - with lane addition

Merge Analysis - with lane addition was posted on April 7th, 2016 at 4:52 PM EDT
regarding Chapter 13: Freeway Merge and Diverge Segments

can you use the HCS software to determine the freeway merge where the on-ramp is an lane addition?

For example: if the freeway section through the interchange is 3 lanes and there is a one lane on-ramp that is a lane addition. What do you assume for N in the software for number of lanes in freeway segment? Is it 3 or 4?

Bill Sampson
University of Florida
RE: Merge Analysis - with lane addition was posted on April 7th, 2016 at 5:04 PM EDT

This situation was clarified in January 2013 (see below) to analyze the segments before and after the on-ramp as basic freeway segments, then check the capacity of the on-ramp using Exhibit 13-10.

Page 13-22 (Clarification, approved January 2013): Add the following new subsection following the first paragraph of the “Special Cases” section: Single-Lane Ramp Additions and Lane Drops

On-ramps and off-ramps do not always include merge and diverge elements. In some cases, there are lane additions at on-ramps or lane drops at off-ramps.

Analysis of single-lane additions and lane drops is relatively straightforward. The freeway segment downstream of the on-ramp or upstream of the off-ramp is simply considered to be a basic freeway segment with an additional lane. The procedures in Chapter 11 should be applied in this case.

RE: Merge Analysis - with lane addition was posted on April 7th, 2016 at 5:20 PM EDT

Thank you!

We have another situation where the freeway segment is 3 lanes in EB direction before merge and at the merge point, the ramp is a 2-lane on-ramp with the outside ramp lane merges as shared freeway/ramp and inside ramp lane is a lane addition that continues for more than 1 mile.

Would you use the HCS 2010 software for this or apply the merge equations from HCM 2010?

If HCS software, what would you assume for number of freeway lanes – 3 or 4? And what would you assume for the number of ramp lanes?

If using equations then which one in the manual?

This situation is a major merge but in reality the on-ramp is not a major roadway. So I am not sure if we can consider it as a major merge.

Please advise on what methodology should be applied in this case.

thanks again,

RE: Merge Analysis - with lane addition was posted on April 19th, 2016 at 7:10 AM EDT

I have usually used the 2 lane ramp methodology for this. Please take number of lanes N as 3 and assume that the both your ramps lanes will merge back onto the freeway. And keep the length of La1=500ft, La2 =500ft to keep the effective length down to 1500 ft. The density estimated will be on the conservative side as you have assumed the outer lane, merges back rather than continuing as a lane addition. Trust this helps.

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