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Precursor Systems Analyses of Automated Highway Systems Resource Materials AHS Entry/Exit Implementation
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1995-11-01
[PDF-1.29 MB]
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Abstract:Entry/exit is one of the major components of highway transportation service. Some might say it is the most important component since it ties directly to origin and destination (OD) pairs, as airline service is tied to city pairs and airport capacity. Entry/exit capacity can dictate a freeway system capacity. As we increase the freeway cruise lane capacity, demand increase can overload the entry/exits. Local street volume in the vicinity will at some point reach capacity. However, automation gives a new tool to deal with system overloads. The traffic controller can directly control sector speed and spacing. As we see in this chapter, the relationship between speed and "safe" capacity contains an optimum, much as manual traffic achieves today, only it is higher and peaks at a higher speed. The controller can choose to modify cruise speed, for increased capacity near an entrance region, to provide more space in the lane for a temporary increase in entry flow. Up to the capacity of the entry procedure, the need for queues or entry lane slowdowns can be eliminated. Entry/exit concept definitions are closely tied to our RSC definitions. In I2, there is a shared or dedicated lane from the manual lanes to the AHS cruise lane. In I3, this transition lane is a dedicated lane and it can originate from a local street. In I2 the lane is not dedicated to AHS vehicles exclusively when participation1 is not high enough to warrant. In I1 the cruise lane is not dedicated for the same reason. Because low participation is associated with the early years of AHS deployment, the RSCs and their corresponding entry/exit concepts have an evolutionary interpretation. Entry/exit is also tied to the RSC communication aspect. As discussed in this chapter, the entry/exit procedures we envision involve predominantly the vehicle/vehicle (VV) communications link and C1 concepts in I1, the roadside/vehicle (RV) communications link and the VV link in I3, and a less complex RV link in I2, with a fully utilized VV link. We feel confident that we can achieve higher vehicle densities with automation. However, if this brings higher person-miles traveled by attracting more and longer personal trips, real increases in travel efficiency are questionable. If, through various measures, we can keep vehicle-miles traveled VMT unchanged and, in addition, the total flow in all cruising lanes is not changed, then maximum flows at entry/exits are not changed and existing ramps and local streets are not overloaded. The benefits to the individual user are shorter and more reliable trip time, assuming that cruising lane congestion was the problem in the first place. If, indeed, entry/exit capacity is a problem, it seems that, short of building more concrete infrastructure, aspects of Intelligent Transportation Systems (ITS) other than vehicle automation must be emphasized to solve congestion problems. There are concepts such as alternate routing and departure time specification, recognizing that everyone cannot use the same portion of concrete at the same time. This line of thinking leads us to examine rearrangement of flow from manual to automated in addition to producing high automated flows. Finally, it seems reasonable to anticipate, with the increasing presence of automated vehicles, an "automation" mind set beginning to dominate all driver behavior. Perceiving automated vehicles to be a benefit to the manual vehicles in terms of decreased congestion and trip time, manual drivers would develop the cooperation and approval needed to share the road with automated drivers. In what follows, the entry/exit techniques can easily be foiled by irresponsible or uncooperative manual drivers. If our transportation systems and ourselves behave intelligently, we will all get to our destinations on time.
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Content Notes:Task J
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