While the five Top Tier dual mode concepts share a number of basic characteristics, they also vary widely as a function of their fundamental design principles. One characteristic common to all the Top Tier concepts is to require a secure environment for automated operation, typically through guideway elevation. The federal government's research on automated highway systems in the 1990's focused on barrier-separated surface lanes for which it is difficult to protect effectively against unauthorized vehicle or pedestrian entry, or from animals and debris. Grade separation permits tight access controls to be focused at the limited number of entry and exit points; the rest of the guideway is comparatively inaccessible.
Given the need for an elevated guideway, three primary trade-offs can be studied to understand the evolution of a particular system design: cost-benefit, technology innovation, and market acceptance. An example of the first, cost-benefit, is the relationship between the cost of elevating the guideway and the weight of the loaded vehicles. The lowest possible gross (loaded) vehicle weight (GVW) can result in the lowest guideway elevation costs, along with such benefits as better fuel economy, aerodynamics and reduced emissions. However, the weight limit may overly restrict those vehicle features (space, human and cargo capacity, off-guideway performance, etc.) that are essential to attract enough public acceptance to be viable. In the U.S. in recent years the average vehicle weight has risen steadily, reflecting a market preference for larger vehicles. This trend runs counter to the need to keep the vehicle weight load low in order to keep the construction costs reasonable. Higher investment in the structures can permit heavier vehicles, but will require higher tolls, reducing customer demand; lower weight limits will reduce costs and tolls, but limit the range of vehicle choice. Choices made regarding cost-benefit trade-offs determine many of a system's design characteristics.
Technology innovation is the second trade-off category: more technological innovation permits expanded dual mode system capabilities with the potential to capture a broader basket of benefits of greatest attraction to drivers. On the other hand, innovations are inherently subject to risk of failure and unanticipated outcomes; compounding innovations in the same system compounds this technological risk. While vehicle automation has been demonstrated in the 1990's FHWA automated highway system project and again more recently in the DARPA Grand and Urban Challenges, no automated system is commercially available. The CEETI study identified some two dozen "critical technologies" (Table 7, p. 35) and rated each of the Top Tier systems for their technology readiness. Interestingly, for any given "critical technology" the "readiness" scores of the different systems were relatively close; however, some critical technology components ranked high (commercially available) while many others ranked low. As noted above, at a minimum a dual mode system must have automated vehicles. The CEETI study shows that some of the critical automating technologies are commercially available while others are in various stages of development. Hence any dual mode design inherently carries substantial technological risk. The system designer must draw a line that includes sufficient technical innovation to create a system that will attract sufficient customers without taking on so much risk that the critical elements of the system fail to function adequately.
The third trade-off relates to risks associated with market acceptance. Introducing a new transportation paradigm – vehicle automation for part of the driving experience – will introduce varying degrees of change from the current driving experience, depending on the design. Changes that result in new benefits, like the elimination of congestion, will attract drivers; however, changes that alter, reduce or eliminate current driving preferences may dissuade them. Market acceptance focuses on the human element of innovation and includes elements such as the driving and riding experience, visual impact of the elevated guideway and noise effects. The challenge is to mitigate these market acceptance risks in the design.