Transportation Engineering on the FE Civil Exam
Transportation engineering accounts for 9 to 14% of the FE Civil exam, making it a critical content area for civil engineering examinees. Topics span highway geometric design, traffic engineering, pavement design, and basic transportation planning. Preparation in this area requires familiarity with design standards, traffic flow theory, and practical applications.
Highway Geometric Design
Horizontal Alignment
Horizontal curves connect tangent (straight) sections of highway. Simple circular curves are defined by their radius (R) or degree of curve (D). Key elements include the tangent length (T), curve length (L), external distance (E), and middle ordinate (M). Know the relationships between these elements and how to calculate them from the radius and deflection angle.
Superelevation (banking of the roadway) and side friction work together to counteract centrifugal force on horizontal curves. The design equation relates the design speed, radius, superelevation rate, and side friction factor. Know how to calculate the minimum radius for a given design speed and maximum superelevation rate.
Vertical Alignment
Vertical curves connect grade lines of different slopes. Both crest (convex) and sag (concave) vertical curves are parabolic. The minimum length of a vertical curve is controlled by stopping sight distance for crest curves and by headlight distance and comfort criteria for sag curves.
Know how to calculate the elevation at any point along a vertical curve using the curve equation, and how to find the high point (on a crest curve) or low point (on a sag curve) location.
Stopping Sight Distance
Stopping sight distance (SSD) is the distance required for a driver to see an object, react, and bring the vehicle to a complete stop. SSD equals the sum of the brake reaction distance and the braking distance. Brake reaction distance depends on speed and perception-reaction time (typically 2.5 seconds). Braking distance depends on speed, grade, and the coefficient of friction between tires and pavement.
Traffic Flow Theory
The fundamental relationship of traffic flow states that flow (vehicles per hour) equals density (vehicles per mile) times space mean speed (miles per hour). The speed-density relationship is typically modeled as linear: speed decreases linearly from free-flow speed at zero density to zero speed at jam density.
The flow-density diagram is parabolic, with maximum flow (capacity) occurring at a critical density. Know how to calculate capacity, critical density, and the relationship between speed, flow, and density for a given roadway.
Level of Service
Level of service (LOS) grades roadway performance from A (free flow, minimal delay) to F (forced flow, significant congestion). The criteria for determining LOS depend on the facility type: for freeways, density is the primary measure; for intersections, average control delay is used.
Intersection Analysis
Signalized intersection analysis involves calculating cycle length, green time allocation, capacity, and level of service. Know Webster's formula for optimal cycle length and the concepts of saturation flow rate, lost time, and effective green time. Understand the terms volume-to-capacity ratio and degree of saturation.
For unsignalized intersections, gap acceptance theory determines the capacity of minor movements based on the gaps available in the major street traffic stream.
Pavement Design
The FE exam covers basic pavement design concepts for both flexible (asphalt) and rigid (concrete) pavements. Understand the concept of equivalent single axle loads (ESALs) for converting mixed traffic to a standard design load. Know the basic layer structure of flexible pavements (subgrade, subbase, base, surface) and the factors affecting pavement thickness design.
For rigid pavements, understand the role of the concrete slab, base layer, and subgrade. Know the basic concepts of jointing (contraction joints, expansion joints, and construction joints) and their purpose in controlling cracking.
Transportation Planning
The four-step transportation planning process includes trip generation (how many trips), trip distribution (where do trips go), mode choice (what travel mode), and traffic assignment (which route). Know the basic concepts and common models used in each step, such as gravity models for trip distribution and the logit model for mode choice.
Study Approach
Transportation engineering problems on the FE exam are generally formula-driven. The FE Reference Handbook provides many of the necessary formulas and design values. Focus your study on understanding when to apply each formula and practicing the calculations. Highway design and traffic flow problems are particularly common and tend to be straightforward once you are comfortable with the formulas.