Answer:
The problems faced in designing a roadway based on a single design vary. The challenges are described in the explanation section below. The first part of the question "If you wanted to create a vehicle that requires a very low tractive effort in order to overcome resistance forces, what parameter would you change and how would you change it" is also answered in italics below
Step-by-step explanation:
When you are designing the roadway geometry you commonly use a design vehicle that gives you the worst scenario viable in that road, which is translated in terms of speed, meaning that after all of the topography problems, localisation of essential regions that can´t be modified (urban regions or national parks for example), ambiental effect and economic issues are handled, the horizontal and vertical geometry goes to depend of the speed value that is going to regulate camber, radius of curvature, acceleration and deceleration longitudes and others.
Now, in terms of infrastructure or pavement engineering, after handling climate challenges, the worst scenario is about weight, that has to do with the total equivalent load that is going to resist that pavement till first failure.So you see, that the challenges of using a single design vehicle in the geometry and infrastructure design of a roadway has to do with the selection of the worst scenario possible.
On the other hand, knowing the LOS of the roadway, which is crucial for knowing how is going to be transit management, depends on evaluating the level of service in all kinds of scenarios, that means different types of vehicles speed and acceleration configurations. So using a single vehicle, in this case, is not going to give you the best real knowledge of the road.
For the first question, designing a vehicle with a low tractive effort means that, from the civil engineering point of view, in order to enhance that vehicle, vertical geometry is key for solving resistance forces, because you can design a road that brings much more speed and power to the vehicle. Friction is also a parameter that reduces speed in vehicles but is necessary for a safe travel so you have see if it is logic to reduce this parameter in order to enhance the design vehicle behavior.
If you want to bring more tractive effort to the vehicle; handling resistance forces caused by rolling resistance or air resistance, drawbar pull and other should be key for enhancing the vehicle behavior. If you want a more detailed answer in this topic you should ask a mechanical engineering expert