Transportation engineering integrates geometric design, pavement engineering, and traffic operations into a coherent system for moving people and goods safely and efficiently. The geometric design of a highway begins with the selection of a design speed, the maximum safe speed that can be sustained over a specific section under favorable conditions. Design speed governs critical elements such as curve radius, superelevation, and sight distance, because every geometric feature must be comfortable and safe at the chosen speed.
Safety along a roadway depends on the driver's ability to perceive hazards and respond. Stopping sight distance is the minimum distance required for a driver traveling at the design speed to perceive an obstacle, react, and brake to a stop, and equals the sum of the brake reaction distance and the braking distance. On horizontal curves, superelevation, also called banking, tilts the roadway cross-section to counteract centrifugal force; in simplified form, e = V²/(gR), with V the vehicle speed, g gravity, and R the curve radius. Adequate superelevation, together with sufficient side friction, allows vehicles to negotiate curves comfortably within the design speed.
Traffic operations are evaluated using concepts from the Highway Capacity Manual. Capacity is the maximum sustainable hourly flow rate that can traverse a roadway section under prevailing conditions of geometry, traffic, and control. Level of Service is a qualitative measure graded from A, representing free-flow operation, to F, representing forced or breakdown flow, considering speed, travel time, freedom to maneuver, and density. LOS provides a common language for evaluating alternative designs and prioritizing improvements.
Pavement design in the United States often follows the AASHTO method, developed from the AASHO Road Test of the late 1950s. The flexible pavement equation relates the structural number, a weighted sum of layer thicknesses, to anticipated traffic loading in Equivalent Single Axle Loads, the resilient modulus characterizing subgrade strength, and acceptable serviceability loss. The California Bearing Ratio expresses subgrade strength as a percentage of the load required to penetrate a standard crushed-rock sample and is widely used for both flexible and modified pavement design. The ESAL converts mixed traffic into a uniform measure by representing each axle configuration in terms of its equivalence to a standard 18-kip single axle load, so that varied truck traffic can be summed into a single design loading.
At intersections, traffic signal warrant analyses evaluate whether installation of a signal is justified, applying criteria from the Manual on Uniform Traffic Control Devices covering traffic volume, pedestrian volume, crash history, school crossings, and other factors. Geometric alignment ties these features together through horizontal curves, circular or transitional arcs that connect tangent sections of the road and are defined by their radius or degree of curvature. Together, design speed, sight distance, superelevation, capacity, level of service, pavement structure, and intersection control form an integrated framework that brings transportation corridors into service.