VIII   ENTRANCE/EXIT STATIONS

 

          In general the station provides the connection to and from the system in both directions; although this may not be universally the case.  Accordingly, the station has two masters; a sector monitor for each direction. The station also provides communication with all carriers within each sector via the station-to-carrier net.  The physical location of both exit and entrance stations will no doubt be the coincident, however as they have different, but interrelated, functions we choose to treat them separately. 

 

          Moreover, it seems likely that an efficient use of real estate will be to designate individual docks as either loading or unloading, not both.  This has the advantage of separating incoming and outgoing traffic, and thus generally providing for the orderly flow of automotive traffic.  In particular, it minimizes any delay caused by an inattentive driver.  This should also provide for the fastest access to or from the system during peak periods, particularly if it is peaked in a single direction.  It has a slight disadvantage in that this approach does not provide for absolute optimum utilization of the carrier (as of necessity it would be empty during transit from unloading to loading) and somewhat compli­cates the shuttling of carriers about the station, but on balance it would seem preferable.  It should be noted that this designation would not necessarily be permanent.  A specific dock may be designated as loading during the morning commute and unloading during the afternoon.

 

         

          A.  Entrance  Station

 

           Each entrance station will have, as a minimum, two entrances.  One for vehicles that have been modified for the system, one for those that have not.  We consider first those that have been modified.

 

          a.  Vehicles Equipped for the System  Upon entering the station, each vehicle is interrogated for a functioning carrier-to-vehicle communication port.  This communication port has the dual role of communicating with the station upon entering and communicating with the carrier after loading.  Satisfied that it has the requisite port, the station instructs it to ask for the vehicle's destination.  We anticipate that the communications between the user and the station will be simple voice commands.  This will facilitate continuous movement of the vehicle through the station.

 

          Having obtained the destination, and what other information it requires, and assuming space is available on the system; the station, via the communication port, will instruct the driver to a specific loading dock where a carrier should be waiting.  A loading dock is simply a means of level access allowing the vehicle to im­mediately drive end-on onto the carrier.  It is anticipated that a given station will require several parallel docks. 

 

          As soon as the wheel chocks have secured the vehicle, and with the con­currence of the station; the carrier moves away from the loading dock toward the appropriate launching track.  While proceeding toward the launching track, the carrier asks the driver for confirmation of the destination, and perhaps shares identity with the vehicle.  The driver need not be involved with the latter.  Upon reaching the launching track, the carrier is launched onto the system.

 

          In busy stations, manifold loading docks may complicate movement to the launching track.  Although carriers possess the means to switch from one track to another, it may prove more effective for stations to direct a carrier by means of electrically coded cables strung under the surface.  Carriers are instructed to follow a particular code.  Moreover, to obviate the danger of electrical contact, an onboard battery provides the power for movement within the station.  Electrical connection to the power grid is reestablished at the launching ramp.

         

          As detailed in section on the unit carrier, the specific means for communica­tion between the station and carriers within the station as well as those on the adjacent sector would have to be by radio.  As detailed in Section VI, the unit carrier, one would imagine that efficient use of spectrum would require something not unlike the present digital cellular telephone system.

 

          b. Vehicle Not Especially Equipped And finally, we must also accommodate those vehicles not appropriately adapted for the system.  These would enter the station through a separate entrance.  A computer screen would inform the driver how to navigate the system, how to obtain a portable control box, and how to plug it in to the carrier.  After that, it would proceed as described above. 

 

 

          B.  Exit Station

 

          The function of the exit station is to receive the carrier from the separation track.  It directs the carrier to a vacant loading dock, the restraining devices are automatically released, and the vehicle departs. 

 

          At this juncture, depending on the immediate needs of the system, a decision is made as to whether the carrier remains at the station or is directed to the launching track for an empty transit to another location.  All this "to-ing" and "fro-ing" requires close coordination with the entrance station, so in this respect our separation of the station functions may be a tad artificial.

 

 

          C.  Station Capacity

 

          In the same sense that a maximum theoretical capacity for the main line was developed, it is useful to consider a similar concept for the maximum capacity of an individual station.  Separate docks as well as separate tracks for incoming and outgoing carriers will minimize interference and congestion for these functions.  In the following, although we describe only a departure system, the requirements of an incoming system are essentially the same.  In most instances, the number of docks will limit the capacity of an individual station. 

 

          For example, if we assume a loading-dock cycle time of 12 seconds; then with 4 docks, the maximum capacity is 1200 vehicles per hour.[1]  We are not limited to this; additional docks will provide additional capacity.  It serves only as an illustrative example.

 

          There are however some limitations imposed by the launching procedure.  In previous examples we have assumed a packet frequency of one every 4.25 seconds, or 847 per hour.  If we limit launching to one per packet, we are limited to that number.  However, there is no reason to do this.  We can form "mini-packets" of 2 or 3 and launch as previously described; thus providing capacity exceeding 2000 vehicles per hour.  A word of caution.  At slow station speeds, let’s say 8 miles per hour, it would be difficult for a single line to supply the launcher at a rate much exceeding 1600 carriers per hour.  The solution is straightforward - two parallel lines fed from separate sets of docks.  These need not be separate all the way to the main line, only as long as necessary to reach a velocity to accommodate the traffic.

 

          It should be noted, however, that a capacity of 2000 vehicles per hour greatly exceeds the ramp traffic of all but a handful of the thousands in Caltran District 7.  Further, we estimate[2] that less than 5 percent of these would not be accommodat­ed by a capacity of 1200, and most can be accommodated by less.  Of this 5 percent, the overwhelming majority can be satisfied with a capacity of 1600 vehicles per hour.  It should also be remembered that it is not intended, nor would it necessarily be a good thing, to absorb all freeway traffic.

 

          The salient message is that adequate station capacity can be provided.  While clearly requiring more space than a conventional freeway ramp, in most instances the real estate requirements are not extensive.             

 

          Implicitly, the foregoing assumes that the supply of carriers is adequate for the demand, and that the number of incoming carriers is equal to the outgoing.  If we were to rely totally on revenue traffic (i.e., a vehicle-carrying carrier) this would appear to be a somewhat unlikely scenario.  However, as previously indicated, empty carriers would be re-cycled and shuttled about to provide the necessary input.  This coordination is an integral part of the operating scenario.   As discussed in Section V, the problem is not as serious as one might expect.  Although the use of auxiliary storage tracks or "carrier barns" at strategic locations will no doubt have to be considered, it should not prove a major factor.  In any event, huge storage areas of carriers left idle after the morning commute awaiting the afternoon commute are simply not in this picture.

 


        [1] The Caltran (California Department of Transportation) design manual indicates that a single-lane metered on-ramp can accommodate 900 cars an hour; or one every 4 seconds.  In this instance, there is the additional burden of moving an empty carrier into position, and clearing the dock.  As neither of these latter involve human responses, if we allow 4 seconds for each, an estimate for the entire process would to be 12 seconds.  Accordingly, 4 docks will provide an average departure rate of one every 3 seconds, or 1200 vehicles an hour.  Even the assumption of an 18 second cycle requires only an additional 2 docks to  provide the same capacity.

 

     [2] The available ramp data are average daily values.  (1998 Ramp Volumes of the California State Freeway system, Districts 7 and 12; State of California; Business, Transportation, and Housing Agency; Department of Transportation; September 1999.)  Of interest, however, are peak values.  We gain some insight into peak to average ratios by considering freeway K values (i.e., the percentage of the average daily rate in both directions during the peak hour).  If we exclude low traffic locations, these typically are in range of 6 to 8 percent (a value of 8.33 percent represents of peak to average value of 2).  Accordingly, a standard value of 2 was adapted for all ramp data.  While not rigorously correct, it is probably terribly wrong either.  Applying this factor to the average daily ramp volumes, and excluding inter-freeway interchanges, we proceed to the conclusions.