Greenshields' model describes a linear relationship between speed and density under uninterrupted flow. In this model, as density increases, speed falls linearly, with the speed at zero density equal to the free-flow speed and speed dropping to zero at a maximum jam density. Mathematically, v = vf (1 − k/kj), where vf is the free-flow speed and kj is the jam density. This straight-line behavior for speed versus density is what identifies the model. The other concepts don’t describe this linear speed–density link: saturation flow rate relates to the maximum vehicle passage rate at a signalized intersection, not a speed-density relationship; a phase is a term from traffic signal timing; and arterial highway refers to a road type rather than a speed-density model.

Transit-Oriented Development focuses development around public transit stops to maximize accessibility and encourage walking. By placing housing, jobs, and amenities within a short walk of rail or bus stations, neighborhoods become easy to navigate on foot or by transit, which reduces the need for car trips and promotes shorter, more convenient journeys. Typical features include higher density near stations, mixed-use development, a connected street layout, and pedestrian-friendly design with strong safety and accessibility. This approach creates a more vibrant, walkable environment and supports higher transit ridership because everyday destinations are reachable without long car trips. Other concepts might address traffic flow, highway speeds, or evaluating environmental effects, but they don’t describe structuring land use around transit to boost walkability and shorter trips in the same integrated way.

NEPA is about making the environmental consequences of federal actions open and clear before decisions are made. The key idea is that agencies must analyze how a proposed action could affect the environment and share that information with the public. This analysis usually takes the form of an environmental assessment for actions with smaller or uncertain impacts, or a full environmental impact statement when the effects are significant. An EIS goes beyond simply identifying effects; it describes the environment being affected, lays out reasonable alternatives (including the no-action option), assesses the potential impacts of each alternative, and proposes mitigation measures. Public review and participation are also integral parts of the process, ensuring transparency and informed decision-making. This approach helps ensure that environmental considerations are part of the decision from the start and that stakeholders can weigh trade-offs before proceeding. The other options don’t fit NEPA because the act does not require funding projects, mandate minimizing all impacts regardless of cost, or establish a certification program for restoration projects.

Peak Hour Factor is about how traffic is spread out within the hour. It’s defined as the hourly volume divided by four times the largest 15-minute volume observed in that hour. If the traffic is evenly distributed, the busiest 15-minute period would be about a quarter of the hourly total, giving a PHF close to 1. If most of the traffic occurs in a short subinterval, the largest 15-minute block is larger relative to the hour, and PHF drops below 1. So a higher PHF means more uniform flow throughout the hour, while a lower PHF indicates a more peaked, uneven distribution. This concept is not about speed, signal timing, or headways; it specifically captures the distribution of flow over the hour. For example, with an hourly volume of 1000 vehicles, a busiest 15-minute block of 250 yields PHF = 1, while a busiest 15-minute block of 400 yields PHF = 1000 / (4*400) = 0.625.