Roadway and Street Design
Roadway and Street Design
Roadway design encompasses geometric design (horizontal and vertical alignment), cross-section elements, intersections, and drainage. Candidates must understand design speed, sight distance (stopping, passing, decision), horizontal curves (radius, superelevation), vertical curves (crest and sag, K-values), lane widths, shoulders, sidewalks, and access management. Key references include AASHTO’s Green Book and local street standards.
Traps include: confusing stopping sight distance (SSD) with passing sight distance (PSD). SSD is the distance required for a driver to stop after seeing an object; PSD is for passing on two-lane roads. Another trap is superelevation: it is the banking of curves; maximum superelevation rates are typically 4-8%. Also, K-value for vertical curves is the length of curve per percent change in grade; for crest curves, K = L/A, where A is the algebraic difference in grades (%). A common error is to use the wrong K for the design speed. Clear zone requirements also appear: the clear zone is the unobstructed area beyond the edge of travel lane, based on speed and traffic volume.
Worked example: A crest vertical curve connects a +3% grade to a -2% grade. If the design speed is 50 mph and the required stopping sight distance is 425 ft, what is the minimum length of curve? First, A = |(+3) - (-2)| = 5%. From AASHTO, for 50 mph, the K value for SSD is about 84 (for crest). Then L = K × A = 84 × 5 = 420 ft. Since 420 ft is less than the SSD of 425 ft, the curve length should be at least equal to SSD? Actually, the formula L = K×A assumes L > SSD. Here L=420 < 425, so we must use the alternative formula for L < SSD: L = 2S - (2158/A) where S is SSD. That yields L = 2(425) - (2158/5) = 850 - 431.6 = 418.4 ft. So minimum L ≈ 419 ft. Always check if L > SSD; if not, use the appropriate formula.
Rule of thumb: For preliminary design, use K = 0.01 × V^2 for crest curves (V in mph) as a rough estimate for SSD, but verify with tables.