Case Study 4 New York State Alternate Route 7

Problem 2

Problem 2

Operational analysis of the I-87/Alternate Route 7 Interchange

Problem 2

Sub-problem 2a. What types of analysis should be conducted?

Sub-problem 2b. What are the levels of service in the weaving sections (points A, B, and C)?

Sub-problem 2c.What are the levels of service along the ramp and at the ramp junction for the on-ramp from Alternate Route 7 to I-87 northbound (point D)?

Sub-problem 2d. What is the effect of making geometric improvements to the ramp and at the ramp junction for the on-ramp from Alternate Route 7 to I-87 northbound?

Problem 2a

What segments do you think should be analyzed and what facility types do they represent?

Observations?

Problem 2a

•Ramp junctions?

Observations?

Problem 2a

Possible ramp junctions

• westbound Alternate Route 7 exit ramp to I-87 northbound • westbound Alternate Route 7 exit ramp to U.S. 9 • westbound Alternate Route 7 entrance ramp from U.S. 9 • eastbound Alternate Route 7 entrance ramp from I-87 northbound • eastbound Alternate Route 7 exit ramp to U.S. 9 • eastbound Alternate Route 7 entrance ramp from Alternate Route 7 • southbound I-87 exit ramp to frontage road • southbound I-87 frontage road exit ramp to Alternate Route 7 • southbound I-87 frontage road entrance ramp from Alternate Route 7 • southbound I-87 frontage road exit to I-87 southbound mainline • southbound I-87 entrance ramp from southbound I-87 frontage road • northbound I-87 entrance ramp from State Route 2 and 7 • northbound I-87 exit ramp to eastbound Alternate Route 7 • northbound I-87 entrance ramp from westbound Alternate Route 7 • entrance ramp on I-87 entrance ramp from U.S. 9 • entrance ramp from U.S. 9 to westbound Alternate Route 7 ramp

Problem 2a

True freeway merge or diverge points

• Westbound Alternate Route 7 exit ramp to I-87 northbound (including both a diverge point from Alternate Route 7 and a merge point onto I-87)

• Eastbound Alternate Route 7 on-ramp from U.S. 9 (this is a merge point) • Southbound I-87 entrance ramp from southbound I-87 frontage road (this is

also a merge point)

Problem 2b

• Weaving sections?

Observations?

Problem 2b

Problem 2b

Questions to consider:

• What type of analysis will produce an evaluation of traffic conditions present at the weaving sections?

• What data are needed to analyze the operations of these weaving sections?

• What are the limitations of the models used to analyze the performance of a weaving section?

• Why are these weaving sections considered to be Type A weaves?

• What measures of effectiveness are used to evaluate the performance of a weaving section?

• What is meant by the terms "constrained operation" and "unconstrained operation?"

Problem 2b

• What type of analysis will produce an evaluation of traffic conditions present at the weaving sections?

• Operational analysis• Design analysis

Problem 2b

• What data are needed to analyze the operations of these weaving sections?

• the freeway free-flow speed• the number of lanes in the weaving section• the length of the weaving section• the terrain topography (level or rolling)• the configuration (type) of the weaving section• the weaving volumes• the peak hour factor• the percentage of heavy vehicles present in the traffic

stream

Problem 2b

• What are the limitations of the models used to analyze the performance of a weaving section?

•Vw, or the total weaving volume in the section, must be less than 2,800 pc/hr for Type A weaves, for example•The flow rate through the weaving section must be less than that allowable for a basic freeway segment.•VR, or the proportion of the total flow that is weaving, must be less than 0.45 for a three-lane section and 0.35 for a four-lane section. •The total length of the weaving section must be less than 2,500 feet, or the section should be considered only as unconnected merge and diverge (ramp junctions) points.

Problem 2b

• Why are these weaving sections considered to be Type A weaves?

• Each weaving traffic stream must make one lane change to reach its desired destination

Problem 2b

• What measures of effectiveness are used to evaluate the performance of a weaving section?

• Traffic density

Problem 2b

• What is meant by the terms "constrained operation" and "unconstrained operation?"

• The number of lanes that must be used by weaving vehicles to achieve equilibrium or unconstrained operation (Nw) and

• The maximum number of lanes that can be used by weaving vehicles for a given configuration (Nwmax).

Problem 2b

Problem 2b

• How will these weaving sections perform, given these inputs?

• What parameters do we use to characterize the performance of the weaving sections?

Problem 2b

Observations?

Problem 2b

• How does the length of each weaving section affect its operation?• The volume ratio, VR, is more than twice as high in Weaves B and C

as in Weave A; is this significant and if so, why?• What is the significance of the predicted speeds for the weaving and

non-weaving traffic? The weaving speeds are approximately 16 to 18 mi/hr less than the non-weaving speeds for five of the time periods presented in the table; is this important and if so, why?

• Why is the weaving traffic constrained? What is the practical implication of this finding?

• What happens when the weaving flow rate exceeds the model limit?• In Weaves A and B, the volume ratio, VR, exceeds the model limit;

what is the likely result that you would observe in the field?

Problem 2b

• As you consider the data, how would you summarize the operations of the weaving sections in this interchange?

Observations?

Problem 2b

• Weave A• LOS A and B • High speed differential between the weaving and non-weaving

vehicles, but proportion of weaving traffic (volume ratio) is low (0.31 and 0.30, respectively) during the two time periods: overall speed of all vehicles in the section is over 50 mi/hr and the resultant densities (9.4 pc/mi/lane and 12.5 pc/mi/lane) are low

• Weave A will operate at acceptable level; no reason to consider changes to the design

• All model limitations are met, so we can be reasonably confidant of our conclusions

Problem 2b

• Weave B• LOS B and E • Very high volume ratio (VR) during both periods, but lower total

volumes mitigate this condition in AM. • In PM , high volume ratio combined with the high overall flow rates

result in an overall speed of 31.7 mi/hr in the weaving section and a density of 38.2 pc/mi/lane

• Volume ratio and total weaving volume model limits are exceeded during PM resulting in a breakdown of operations and queuing at some locations.

• Volume ratio limit exceeded in AM, again resulting in poor operations.

• Conclusion: even though the forecasted LOS is B for the AM peak, both time periods will experience poor operations with breakdowns in flow to be expected

Problem 2b

• Weave C• LOS E and F• With high density and the failing conditions we should consider

another tool in addition to the HCM, such as a microscopic simulation model, to evaluate this weaving section

Problem 2c

• Should we consider only the vicinity of the junction itself, or are there other areas that we should consider as well?

• What input data are needed to conduct this analysis?• What is the primary measure of effectiveness for a merge point

analysis?• What parameters are forecasted by the merge point analysis models

in the HCM?• What are some of the limitations of the merge point analysis model

that we must keep in mind when applying it to this sub-problem?

Problem 2c

• Should we consider only the vicinity of the junction itself, or are there other areas that we should consider as well?

• Merge influence area• Merge with US 9 ramp• Capacity of ramps

Problem 2c

• What input data are needed to conduct this analysis?• number of lanes on the freeway• free flow speed on the freeway• freeway volume just upstream of the merge point• free flow speed of the ramp• ramp volume• number of lanes on the ramp• length of the acceleration lane(s)

Problem 2c

• What is the primary measure of effectiveness for a merge point analysis?

• Density (expressed in terms of vehicles per mile per lane)

Problem 2c

• What parameters are forecasted by the merge point analysis models in the HCM?

• flow rate in the merge influence area • density in the merge area• level of service • speed

Problem 2c

• What are some of the limitations of the merge point analysis model that we must keep in mind when applying it to this sub-problem?

• it does not apply when demand exceeds capacity• If demand exceeds capacity, we need to consider another

procedure, possibly microscopic simulation

Problem 2c

Problem 2c

• What is the significance of the parameter, PFM?• Which data from the table above describe the nature of the flow of

traffic in the merge influence area?• What is the basis for the forecast of level of service?• How would you describe the operation of the merge point in the PM

peak period?

Problem 2c

• What is the significance of the parameter, PFM?• Important function of the merge point analysis: estimate the lane

distribution of traffic in the vicinity of the merge point• PFM: proportion of the approaching freeway flow remaining in

lanes 1 and 2 immediately upstream of the merge point • PFM depends on number of lanes on the freeway mainline and

length of the acceleration lane from the ramp to the mainline.• PFM = 0.59; 59 percent of the approaching freeway flow remains in

lanes 1 and 2 immediately upstream of the merge point • If the freeway had more lanes at this point, this value would be

lower, as more of the mainline traffic would avoid the turbulence of the merge area

Problem 2c

• Which data from the table above describe the nature of the flow of traffic in the merge influence area?

• Flow rate in the merge influence area (vR12) is compared to the capacity of the area to determine whether this area is under capacity or over capacity

• During PM, vR12 exceeds the capacity of the merge influence area

• significant part of the demand (5,264 - 4,600 = 664) can't be served during the analysis period

• unserved demand is diverted to the next analysis period

Problem 2c

• What is the basis for the forecast of level of service?• Density is used to determine level of service• PM peak, 42.4 pc/mi/lane (LOS F) • AM peak, 13.7 pc/mi/lane (LOS B)

Problem 2c

• How would you describe the operation of the merge point in the PM peak period?

• Operation of the merge point during PM peak period is poor• Demand exceeds capacity, the density is high, and the speed is

relatively low• Since the demand exceeds capacity, we need to consider another

analysis tool, such as a microscopic simulation tool• HCM chapter 25: this methodology does not take into account

oversaturated conditions

Problem 2d

• How can we improve the level of service at the ramp junction at the

northbound on-ramp to I-87?

• Variables of prime importance

• Free flow speed on the ramp

• Free flow speed on the mainline

• Number of lanes on the ramp

• Number of lanes on the mainline

• Length of the acceleration lane in the merge area

• What possible design changes would you suggest to mitigate the

problem at the ramp junction?

Observations?

Problem 2d

• What possible design changes would you suggest to mitigate the

problem at the ramp junction?

• adding a lane to the freeway mainline

• adding a lane to the ramp

• lengthening the acceleration lane

Problem 2d

• Adding a lane to the freeway mainline• What is the most

significant change in the performance of the merge area as a result of the addition of the lane to the freeway mainline?

• Does the addition of the lane produce acceptable performance of the merge area?

Problem 2d

• Adding a lane to the freeway mainline• What is the most

significant change in the performance of the merge area as a result of the addition of the lane to the freeway mainline?

• Does the addition of the lane produce acceptable performance of the merge area?

Problem 2d

• Adding a lane to the ramp

• What is the most significant change in the performance of the merge area as a result of the addition of the lane to the freeway mainline?

• Does the addition of the lane produce acceptable performance of the merge area?

Problem 2d

• Lengthening the acceleration lane

Problem 2 analysis

Problem 2 discussion

Discussion points• We don't have results that are completely reliable.

• For two of the weaving sections and for the ramp junction, the model boundaries were exceeded, thus putting some uncertainty on the results produced by the HCM analysis.

• While we have looked at the components of the interchange, we are still conducting this analysis in some isolation.

• How does the operation of the interchange affect the mainline section of I-87, particularly in the vicinity of the various ramp junctions?

• How does the interchange affect the operation of Alternate Route 7?