Problem 5: Interstate 87 Interchange

We will show how to apply the HCM procedures to study several different points within the interchange.

5a: Freeway merges and diverges

5b:Route 146 weaving section

5c: Stop-controlled intersections

Characteristics of the I-87 Interchange

Partial cloverleaf with 3 on-ramps and 3 off-ramps

SB there is: single-lane off-ramp (direct) leading to

Route 146 W single-lane off-ramp (loop ramp)

leading to Route 146 E double-lane on-ramp (semi-direct) for

both WB and EB traffic on Rt 146 NB there is:

Double-lane off-ramp. It expands to a triple-lane ramp then becomes 4 approach lanes (2 leading WB and 2 leading EB)

single lane on-ramp (loop) from Route 146 EB

single lane on-ramp (direct) from Route 146 WB

Sub-problem 5a: I-87 Interchange - Merges and Diverges on the Freeway

What parameter defines level-of-service at a ramp junction?

The density of vehicles within the ramp influence area.

Where is the ramp influence area?

It is the downstream/upstream area within 1,500 feet of the merge/diverge point, respectively, and includes the two right-most lanes.

Observations?

Single Lane Ramps

southbound-to-westbound off-ramp

AM Existing condition: Freeway volume upstream = 3,751 vph Ramp volume = 437 vph The downstream off-ramp volume = 314 vph

AM Without condition: Freeway volume upstream = 3,841 vph Ramp volume = 455 vph The downstream off-ramp volume = 326vph AM With condition: Freeway volume upstream = 3,851 vph Ramp volume = 565 vph The downstream off-ramp volume = 326vph

Case Density Speed LOS

AM Existing 23.2 54 C

AM Without 23.9 54 C

AM With 24.6 54 C

I-87 Interchange Southbound-to-Westbound Off-Ramp

How does this ramp perform?

LOS = C (all cases)

Multiple Lane Ramps

How would a second lane affect ramp performance?

Case # Lanes Density Speed LOS

PM Existing 1 23.2 52 C

PM Existing 2 9.3 52 B

PM Without 1 24.1 52 C

PM Without 2 10.2 52 B

PM With 1 24.1 52 C

PM With 2 10.2 52 B

Exhibit 2-57. I-87 Interchange Northbound Off-Ramp

The densities for a single lane are more than twice that of a double lane ramp!!!

Observations?

Sub-problem 5b: I-87 Interchange: Route 146 Weaving Section

In the HCM what are the input variables for analyzing weaving sections? Type of weave (A, B, or C) Length of the weaving

section Free-flow speed Terrain The 4 weaving volumes Peak hour factor Percent trucks Percent recreational vehicles

Case Density Speed LOS

AM Existing 14.69 43.95 B

AM Without/With 15.35 43.69 B

PM Existing 22.96 43.68 B

PM Without 24.06 43.38 C

PM With 26.43 41.74 C

Exhibit 2-60. I-87 Interchange Performance for the Eastbound Weave on Route 146

Performance of the Route 146 weaving section under a variety of conditions

Prediction of Delays at the Terminus of the Southbound-to-Eastbound Off-ramp

How realistic is it to assume a critical gap of 6.2 seconds for right-turning vehicles?

Case Delay v/c 95% Q LOS

PM With (6.2 sec)

31.7 0.66 4.56 D

PM With (4.1 sec)

15 0.41 1.98 B

Exhibit 2-61. I-87 Interchange Prediction of Delays at the Terminus of the Southbound-to-Eastbound Off-Ramp

These results are not consistent with field conditions, therefore the critical gap must be much shorter

How realistic is it to assume a critical gap of 4.1 seconds for right-turning vehicles?

These results are closer to observed field conditions

It is necessary to use an accurate critical gap value!!!

Sub-problem 5: I-87 Interchange – stop-controlled intersections

Case Delay v/c 95% Q LOS Note

AM Existing 29.6 0.78 7.38 D

AM Without 35.2 0.83 8.71 E

AM With 74.3 1.04 16.29 F

AM With Modified

31.2 0.84 9.52 D CG=4.1 sec

PM Existing 86.4 1.01 11.6 F

PM Without 115.7 1.1 13.88 F

PM With 243.4 1.44 24.95 F

PM With Modified

66.5 0.98 12.8 F CG=4.1 sec

Exhibit 2-62. I-87 Interchange Performance of the Southbound-to-Westbound Off-Ramp

What might be done to mitigate the LOS F predictions?

What have we learned?

The interchange is adequate with the site-generated traffic, except for the SB-to-WB off-ramp and the WB left onto the SB on-ramp.

In the context of the HCM procedures, we’ve seen how a variety of methods can be used to explore the performance of specific locations within the interchange, including: Merge and diverge analyses to the major ramp

terminals Weaving analysis of the EB section of highway

between the two loop ramps Use of the unsignalized intersection analysis

methodology to assess the performance of two stop-controlled intersections