SECTION III Service Systems
8.9 Using Economic Principles to Guide Ergonomic Studies of Automated Highway Design
On occasion, ergonomists are called upon to help develop new public systems or to assist in creating new laws regulating industrial operations, usually in health and safety. Those situations usually do not have easy precedents to follow. The only other recourse is for ergonomists to follow reasonableness and evolution. There is perhaps no better guide than selecting the less costly alternative as a starting point.
The case below involves the design of a new form of highway system where the conceptual design is uncertain. Using the reasonableness and evolution principle, it was assumed that the new system would be as close to our existing systems as was possible without jeopardizing the fundamental nature of the concept.
For a couple of decades, highway people have dreamed of having an automated highway system (AHS) where a computer could control the vehicles. Part of this dream was a fantastic reduction in automobile accidents. Another part was an equally fantastic increase in highway capacity particularly around our large cities where huge numbers of drivers enter the city in the mornings and leave each evening. It is even more obvious that the urban AHS also has the problem of very expensive land so that conventional expressways cannot be economically used to solve the problem of excessive traffic volumes. However, it is even less clear what alternative design of an AHS is even reasonably feasible. The story to follow is a
FIGURE 8.3
xi= 1 2 3 4 5 6 7 8
y1(xi) = 33.0 36.9 42.0 48.6 57.1 68.3 82.8 101.6
y2(xi) = 43.0 46.6 50.9 56.1 63.3 69.8 78.8 89.5
result of an initial effort by the Federal Highway Administration (FHA) of the U.S. government in an ergonomic research effort to explore this question. (See Buck and Yenamendra, 1996.)
In an initial examination of the highway economics by the AHS research group, it became evident that it might be possible for a relatively low-cost version of the AHS concept to work and so this group of researchers first focused on that concept. If the initial experimentation proved otherwise, then an alternative AHS with greater investments and expenses could be examined. Although we are referring to the initial version as low-cost, that is only relative to the other versions. Costs of this version will be substantial because of extensive computers and sensor equipment, as well as at least one added traffic lane on the existing expressways and bridges. The envisioned concept was that the traditional two-lane (each way) expressway would be expanded by a single third lane in each direction which would be used exclusively by AHS-outfitted vehicles. This added lane is expected to cost at least $3 to $4 × 106 per mile in each direction for just the concrete. While this configuration is not inexpensive, it is vastly less expensive than duplicating the existing interstate roadways as an alternative plan proposed.
Figure 8.4 illustrates this AHS concept with strings of AHS vehicles traveling in the left-most lane with gaps between successive strings. The right two lanes contain non-AHS vehicles and AHS vehicles either waiting to join the other AHS vehicles, departing from the AHS lane for an exit ramp, or AHS vehicles who simply choose to remain in manual mode for one reason or another. That schematic in Figure 8.4 illustrates the entering AHS vehicle coming from the on-ramp to the right lane of the expressway, crossing over to the center lane before attempting entry into the AHS lane. It was assumed that the entry procedure required the driver to first request entry, then the central computer would check the vehicle for adequacy to drive in AHS mode, and if the checkout was adequate, the computer would direct the vehicle to enter immediately behind a passing string of AHS vehicles. The driver of the entering vehicle was instructed to accelerate as rapidly as possible after entry until the computer took over. One of the modes of transferring control from the driver to the central computer was automatic as soon as the first wheel crossed into the AHS lane. The other mode was where the driver manually shifted control by pushing a button on the steering wheel. It was felt that there may be legal reasons for a manual mode of control transfer even though that mode may be slower.
It was assumed that AHS cars left the AHS lane by requesting to exit, splitting the string of vehicles so that the exiting vehicle was last, and then manually driving into the center lane. Experiments in an automobile simulator verified that both older and younger drivers could perform the maneuvers of both
FIGURE 8.4
8-28 Occupational Ergonomics: Design and Management of Work Systems
entering and exiting AHS. Almost all subjects experienced difficulties during exiting. The difficulties encountered during exiting were almost always due to difficulties in seeing slower manually controlled vehicles in the middle lane due to the vehicles immediately ahead of the exiting car. As the driver of the exiting car was seated on the left-side of the car and exiting maneuvers necessitated rightward movements of the vehicle, another car could be a short distance ahead and not be very visible due to intervening cars. This difficulty was compounded with greater speed differences between the AHS design speeds and those allowed for manually controlled traffic because of the short time between entry and encountering a vehicle in the middle lane. Those difficulties appeared to be correctable with some form of vision aiding.
Entering maneuvers posed a different approach. The maneuvers necessitated the gaps between successive strings of vehicles driving along the AHS lane in order to get the entering vehicle safely into that lane. If the gap was too small, then the entering vehicle could not accelerate rapidly enough to the AHS design speed before the following vehicles would catch up to the entering vehicle. Clearly, the greater the differential in speed between the center lane and the AHS design speed, the longer it takes a vehicle to accelerate to the AHS design speed and the greater the gap needed between successive AHS strings of vehicles. In fact, the entering car was traveling at 55 miles per hour in the center lane and then acceleration to any of three different AHS design speeds of 65, 80, and 95 miles per hour requires respective minimum string-to-string distances of 32.5, 121.5, and 335.0 meters. That is, the gap at the AHS design speed of 80 miles per hour is almost 3.75 times that at 65 miles per hour and the one at 95 miles per hour is 2.76 times the length of the gap at 80 miles per hour or 10.3 times the one at 65 miles per hour. It was envisioned that the controlling computer could create an opening for a new vehicle by slowing down vehicles behind the spot targeted for the entering vehicle to a sufficiently long gap. As the vehicle entered at a lower speed and started to accelerate, the faster vehicle behind would catch up and close the gap. Our experiments on people accommodating with this entry play proved that they could perform the maneuvers extremely well. In fact, the degree to which they could perform the entries came as a surprise.
After the vehicles entered the AHS lane, they were placed in collections of up to four-car strings which would move at the AHS design velocity with this minimum gap. Minimum gaps were about 1/16 seconds and that time interval translated into about 3 to 5 meters separation distance. When those four-car- strings have minimum gaps within strings and inter-string distances as shown above for these three design speeds, the number of vehicles per hour in the AHS lane at this maximum theoretical case would be 7,551, 3,531, and 1,825 vehicles per hour for the AHS design speeds of 65, 80, and 95 miles per hour, respectively. As a result, the lowest of these three AHS design speeds was recommended with this particular design for greater highway capacities. Note that the capacities described above are theoretical upper limits rather than reasonable capacity estimates. But at the recommended AHS design speed, that theoretical capacity is 441% greater than the upper limit of conventional traffic, which is about 1712 vehicles per hour. If the practical upper limit of AHS capacity was only 3/4 of the theoretical limit and there were ample AHS outfitted vehicles to use the single AHS lane, the increase in capacity due to AHS capability would be expected to be over 175% of that with an added third lane of conventional traffic. This economic information would have been impossible without the ergonomic information that people could perform the entry and exit tasks and the delay in AHS traffic they cause during the entry.
These data demonstrate that the AHS configuration is feasible and that this lowest-cost plan can be economically viable, but it does not show that another configuration may be better. For example, another configuration could consist of a partial fourth lane which is added at locations where entry and exit maneuvers are to be performed and that lane would be long enough to allow vehicles to accelerate to the AHS design velocity and decelerate from it to 55 miles per hour. This new configuration would increase AHS capacity more at the higher speeds, but that added capacity is conjecture without confirming ergonomic data. If the ergonomic testing had been carried on assuming a more expensive version of AHS, the public would have insisted on running further ergonomic tests at lower-cost versions of AHS before starting the long political journey toward the AHS. As it is, future ergonomic studies can proceed
to fine-tune the eventual AHS configuration, (e.g., the acceleration lane addition) now that initial feasibility appears evident.