Design Speed
Select design speed and verify it governs curvature, sight distance, and superelevation.
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Highway & Traffic Engineering · Geometric design, traffic analysis, pavement, intersections and roadway safety to AASHTO and MUTCD.
Deliverable: Design speed selection memo with minimum radius check.
Minimum tables, figures and equations for Design Speed
Tables — at least 6
- Table — trial sections or sizes considered, with the capacity of each and the selection decision
- Table — final selected geometry for every element: dimensions, thickness, grade, spacing, elevation
- Table — slab, beam, column and shear wall schedule with governing demand
- Table — ultimate limit state check summary: demand, capacity, ratio, pass or fail, governing clause
- Table — serviceability check summary: deflection, crack width, settlement, freeboard or velocity against its limit
- Table — factors of safety achieved against the factor required, per failure mode
Figures — at least 4
- Figure — free body diagram of each isolated element, fully labelled with loads, reactions, dimensions and axes
- Figure — shear and moment (or pressure and velocity) diagrams for each force-carrying element
- Figure — dimensioned section or plan of each designed element
- Figure — capacity versus demand plot, interaction diagram, or rating curve as applicable
Equations — at least 8
- Equation — equilibrium equations written out for each free body (sum of forces and sum of moments, or continuity and energy)
- Equation — the internal force relations V(x) and M(x), or the momentum/thrust relation, used to compute each element's demand
- Equation — the resulting demand at the critical section of each element, with numeric substitution
- Equation — the capacity expression for each element type, shown with full numeric substitution and units
- Equation — the sizing criterion that sets the final dimension (for example required area, depth or diameter)
- Equation — each limit state check written as demand over capacity with numbers substituted
- Equation — the factor of safety calculation for each failure mode checked
- Equation — punching shear, drift and deflection checks with limits
Number every table and figure (Table 4.x, Figure 4.x), caption it, and refer to it by number in your text. Number displayed equations and show the substitution with units. These counts are minimums — add whatever else your design needs.
Engineering documentation standard — required in every Chapter 4 subsection
These rules are graded on every subsection. Work that misses them is capped on technical accuracy, exhibits, codes and communication, whatever the quality of the prose.
Code and standard references
- Every requirement, factor, coefficient, limit and allowable you apply cites the governing document AND the exact section, article or sub-article number — e.g. ACI 318-19 §22.5.5.1, AISC 360-22 Chapter J, Section J3.6, AASHTO LRFD 10th Ed. Article 3.6.1.2.2, ASCE 7-22 §12.8.1, ASTM D2487, state DOT manual section, local stormwater manual chapter.
- Give the edition or year of every document the first time it appears, then use a consistent short form.
- Where a code equation is used, quote the equation number (e.g. Eq. 22.5.5.1) next to your displayed equation.
- Where you depart from a code provision, state the clause you are departing from and the engineering justification.
- List every code, standard and manual actually used in a Codes and Standards table at the start of the subsection.
Citations for statements
- Every statement of fact, value taken from elsewhere, material property, soil parameter, rainfall depth, unit cost or published method carries an in-text citation (APA) to its source.
- Field and lab data cite the report, boring log, gauge, survey file or test number and its date.
- Manufacturer data cites the product literature and revision date; software results cite the program, version and model file name.
- Uncited assertions are treated as assumptions and must appear in the assumptions table with a justification.
- Every in-text citation resolves to a full entry in the reference list.
Step-by-step calculations
- Structure every calculation the same way: (1) objective, (2) governing code clause, (3) equation in symbolic form with the equation number, (4) definition of each symbol, (5) numerical substitution, (6) result with units, (7) comparison against the limit and the pass/fail statement.
- Show the substitution line — never jump from the formula to the answer.
- Number displayed equations sequentially (Eq. 4.1, 4.2, …) and refer to them by number in the text.
- State the load or flow combination governing each calculation by name.
- Carry consistent significant figures and round only at the reported result; state the rounding convention once.
- Present repetitive element checks in a calculation table with one row per element and the same column order throughout.
Free body diagrams and figures
- Draw a separate free body diagram for each isolated element — no combined sketches standing in for several members.
- Dimension every FBD: span, depth, thickness, cover, eccentricity, embedment, slope, pipe diameter, wall height — with the dimension lines and values shown.
- Label every force, pressure, reaction and moment with its symbol, magnitude and units, and show the sign convention and coordinate axes.
- Show supports and boundary conditions explicitly (pin, roller, fixed, elastic, buoyant, hydrostatic).
- Accompany each FBD with its shear, moment, thrust, pressure or hydraulic grade diagram at the same scale reference.
- Number and caption every figure (Figure 4.x) and refer to it by number in the narrative; add a scale or north arrow to plans.
Units and notation
- Every number in text, tables, figures and equations carries its unit — no bare numbers.
- Use one unit system throughout (US customary or SI); if both appear, give the converted value in parentheses consistently.
- Check dimensional homogeneity of each equation and say so — the units of both sides must match.
- Provide a nomenclature table defining every symbol with its unit.
Checking and verification
- Every calculation is checked by an independent route — hand check against software, alternative method, order-of-magnitude estimate, or a published worked example — and the check is shown, not just claimed.
- Report demand-to-capacity ratios and factors of safety against the required values, with the source clause for each required value.
- Include a verification/checking table: item, method of check, expected, obtained, difference, accept or revise.
- Sanity-check every result (magnitude, direction, plausibility) and state the conclusion.
- Record who checked the work and on what date; flag anything still unverified as an open item.
- State limitations and the range over which the result is valid.
How to complete this section
Do this next: Read the Design Speed lecture and the worked example so you know what "Design speed selection memo with minimum radius check." has to contain.
Not sure how to start or how much depth is expected? Read the fully written model example for this deliverable first — it shows the structure, tables and level of justification your advisor grades against.
Transportation Design Office — what this workspace teaches
Geometric design, traffic analysis, pavement, intersections and roadway safety to AASHTO and MUTCD.
- AASHTO Green Book design controls: design speed, functional class, design vehicle
- Traffic data: AADT, DHV, K and D factors, truck percentage, growth projection
- Horizontal alignment: curve radius, superelevation, side friction, transitions
- Vertical alignment: grades, crest and sag curves, K-values
- Sight distance: stopping, decision, passing and intersection sight distance
- Cross sections: lane and shoulder widths, side slopes, clear zone
- Pavement design: ESALs, subgrade support, layer thicknesses (flexible and rigid)
- Intersection design, roundabouts, turn lanes and capacity
- Traffic signals: warrants, phasing, cycle length, timing (MUTCD/HCM)
- Roadway drainage and safety: crash analysis, barriers, clear zone
End-of-term milestones
- Tuesday, November 17, 2026 — Poster printed and ready. 36 in × 48 in poster finalized and printed one week before the November 24 showcase.
- Wednesday, November 18, 2026 — Final document package uploaded for scoring. Chapters 4–5, calculation package, drawings and appendices uploaded in the app for advisor scoring.
- Wednesday, November 18, 2026 — Poster presentation to faculty and industry. Wednesday poster session — printed 36 in × 48 in poster presented in person; industry reviewers score communication and impact.
- Wednesday, November 25, 2026 — Oral presentation and defense (scored). Scored oral presentation and defense held on Wednesday, November 25.
Design Speed
Select design speed and verify it governs curvature, sight distance, and superelevation.
Section B
Engineering story
A real project situation that frames this module
Week 5: design speed is the item standing between the team and a reviewable roadway geometry, control plan and operational analysis. Select design speed and verify it governs curvature, sight distance, and superelevation. Review stalls on a single line: the team cannot show the record behind design speed vs.
Using posted speed instead of design speed for curve computations. Because design speed selection based on functional class, terrain, and context, the error does not stay local: it is carried into the design of record that drawings, quantities and cost are generated from, and every downstream product inherits it before anyone notices.
Drivers, pedestrians, cyclists, transit riders and the agency that owns the facility carry the consequence. On this module specifically, the exposure runs through speed consistency between successive geometric elements, and the cost of correction rises every week the roadway geometry, control plan and operational analysis moves closer to issue.
Decisions the engineer must make
- What record establishes design speed vs, and is that record in the project data inventory?
- Does AASHTO Green Book (7th Ed.), Ch. 3, govern here — and is that the edition adopted by the jurisdiction?
- What is the acceptance criterion for design speed selection based on functional class, terrain, and context, and was it written before the result was known?
- Is e + f = V²/(15R) valid over the parameter range this project actually occupies?
- If the check fails, does the team revise the roadway geometry, control plan and operational analysis or raise a change request against the locked baseline?
Photo 1. Peak-hour demand: the flow rate that drives capacity, delay and level-of-service analysis.
Wikimedia Commons, CC BY 2.0
Section C
Why this matters
Professional
A licensed engineer defending design speed cites AASHTO Green Book (7th Ed.), Ch. 3, and shows the record behind each input. Your design speed selection memo with minimum radius check. is reviewed the same way — traceability is assessed before arithmetic.
Technical
Design speed vs is what makes e + f = V²/(15R) usable on this project rather than a formula copied from a reference. Get it wrong and every quantity derived from it is wrong by the same factor.
Safety
The failure mode this module guards against is a geometric or control element that puts drivers in a conflict they cannot resolve. It reaches people through relationship of design speed to minimum radius via side friction and superelevation, which is why the safety check is recorded explicitly here rather than inferred from a passing strength or performance check.
Economic
The design of record that drawings, quantities and cost are generated from is priced from this work. Quantities, unit costs and schedule float all trace to design speed vs; a late correction here is paid for as a change order, not a redline.
Environmental
Environmentally, this module fixes vehicle delay emissions, pavement material demand and stormwater from added impervious area. Choosing conservatively without justification is not free — the excess shows up as material, energy and land that the project consumes for no measurable gain.
Community
Everyone who walks, rides or drives the corridor every day inherit whatever this module decides — performance, accessibility, cost of ownership and resilience are set here, not at the ribbon-cutting.
Section D
Learning objectives
By the end of this module you will be able to:
- 1.Apply design speed vs, using this project's own conditions rather than a textbook case.
- 2.Compare design speed selection based on functional class, terrain, and context, using this project's own conditions rather than a textbook case.
- 3.Analyze speed consistency between successive geometric elements, using this project's own conditions rather than a textbook case.
- 4.Justify relationship of design speed to minimum radius via side friction and superelevation, using this project's own conditions rather than a textbook case.
- 5.Compute the governing quantity from e + f = V²/(15R), with a unit audit on every term.
- 6.Apply AASHTO Green Book (7th Ed.), Ch. 3, and cite the section that governs your acceptance decision.
- 7.Reproduce the worked example for design speed = 50 mph, emax = 0.06, fmax = 0.14 and defend the interpretation of the result.
- 8.Produce design speed selection memo with minimum radius check. at a standard the DOT design reviewer would accept without a second revision cycle.
Section E
Instructional content
Full lecture notes with figures and governing equations
Reading design speed as a practising engineer
Select design speed and verify it governs curvature, sight distance, and superelevation. That single sentence hides the substance of the module: design speed vs, and design speed selection based on functional class, terrain, and context. Both must be established from project evidence before anything downstream is credible.
In transportation engineering, this work is the input to the roadway geometry, control plan and operational analysis. Speed consistency between successive geometric elements — which is why this page asks you to record the source of every quantity, not just its value. The design of record that drawings, quantities and cost are generated from depends on it.
- Design speed vs. operating speed (85th percentile) vs. posted speed distinctions
- Design speed selection based on functional class, terrain, and context
- Speed consistency between successive geometric elements
- Relationship of design speed to minimum radius via side friction and superelevation
Photo 1. Reading design speed as a practising engineer in practice — Peak-hour demand: the flow rate that drives capacity, delay and level-of-service analysis.
Wikimedia Commons, CC BY 2.0
Governing relationships and how they are applied here
The relationships below govern design speed. e + f = V²/(15R) — each is valid only inside the parameter range this project occupies, so state that range before substituting.
Design speed selection based on functional class, terrain, and context sets the values you place into these expressions. Any code-prescribed factor must match AASHTO Green Book (7th Ed.); a factor lifted from a different edition silently changes the answer.
e + f = V²/(15R)
- e = superelevation rate (ft/ft)
- f = side friction factor
- V = design speed (mph)
- R = radius of curve (ft)
Photo 2. Governing relationships and how they are applied here in practice — Peak-hour demand: the flow rate that drives capacity, delay and level-of-service analysis.
Wikimedia Commons, CC BY 2.0
Constraints, adopted standards and the safety case for design speed
AASHTO Green Book (7th Ed.), Ch. 3, governs this module: Design speed selection and minimum radius
The safety case is explicit here. The failure mode is a geometric or control element that puts drivers in a conflict they cannot resolve; the people exposed are drivers, pedestrians, cyclists, transit riders and the agency that owns the facility; the control that prevents it is relationship of design speed to minimum radius via side friction and superelevation together with an independent check by someone who did not perform the work.
- Controlling criterion for this module: design speed vs.
- Adopted reference: AASHTO Green Book (7th Ed.) — cite Ch. 3 by number.
- Failure mode guarded: a geometric or control element that puts drivers in a conflict they cannot resolve.
- Evidence produced: Design speed selection memo with minimum radius check..
Photo 3. Constraints, adopted standards and the safety case for design speed in practice — Peak-hour demand: the flow rate that drives capacity, delay and level-of-service analysis.
Wikimedia Commons, CC BY 2.0
Where this method stops being valid
The worked example — design speed = 50 mph, emax = 0.06, fmax = 0.14 — holds only while its assumptions hold. Any horizontal curve on this roadway must have a radius of at least 833 ft to keep side friction demand within the assumed limit at design speed. Outside that envelope the arithmetic still returns a number, and the number is wrong in a way no unit check will catch.
For this project, the boundary you are most likely to push is relationship of design speed to minimum radius via side friction and superelevation. If you cross it, say so in writing, bound the error, and carry the limitation into your results chapter. A disclosed limitation is professional practice; a silent extrapolation is not.
Photo 4. Where this method stops being valid in practice — Peak-hour demand: the flow rate that drives capacity, delay and level-of-service analysis.
Wikimedia Commons, CC BY 2.0
Section F
Engineering workflow
Steps
- 1. Assemble the inputs this module needs — design speed vs; design speed selection based on functional class, terrain, and context — each with a unit and a source record.
- 2. Confirm AASHTO Green Book (7th Ed.) is the adopted edition and locate Ch. 3.
- 3. State the assumptions and the acceptance criterion for design speed vs.
- 4. Evaluate e + f = V²/(15R) term by term, carrying one extra significant figure.
- 5. Test the result against speed consistency between successive geometric elements.
- 6. Audit units and run an order-of-magnitude check by hand before the number leaves your desk.
- 7. Obtain an independent check from a teammate who did not perform the work, and record their name and date.
- 8. Assemble design speed selection memo with minimum radius check. and submit it to the DOT design reviewer for review.
Decision points
- Is every input behind design speed vs traceable? If not — stop and collect the record.
- Does the result satisfy design speed selection based on functional class, terrain, and context? If not — revise the work, never the criterion.
- Would the correction change the design of record that drawings, quantities and cost are generated from? If yes — raise a change-control request before proceeding.
- Have you ruled out the most common error on this module — using posted speed instead of design speed for curve computations?
Quality checklist
- Documented: design speed vs
- Documented: design speed selection based on functional class, terrain, and context
- Documented: speed consistency between successive geometric elements
- AASHTO Green Book Ch. 3 cited by section number
- Units audited on every expression
- Acceptance criterion recorded before the result
- Independent check signed and dated
- Design speed selection memo with minimum radius check. attached and named per the course convention
Section H
Interactive visualization
Design Speed — step-through
Advance one frame at a time. Each frame adds one engineering decision to the previous state.
Step 1 of 6
Select design speed
Section I
Applicable codes and standards
AASHTO Green Book
7th Ed. · Ch. 3
Adopted design/analysis reference governing this module.
Relevance: Design speed selection and minimum radius
Reference the section number and edition in your calculation package. Do not reproduce code text.
Section J
Worked examples
Full engineering solution format
Section K
Common mistakes and how to avoid them
- Using posted speed instead of design speed for curve computations
- Ignoring speed-consistency checks between adjacent curves
- Treating design speed vs as a given instead of establishing it from a project record.
- Producing design speed selection memo with minimum radius check. without showing how design speed selection based on functional class, terrain, and context was satisfied.
- Substituting into e + f = V²/(15R) outside the range where it is valid, and reporting the number anyway.
- Missing relationship of design speed to minimum radius via side friction and superelevation, which is exactly the path to a geometric or control element that puts drivers in a conflict they cannot resolve.
- Designing to the average condition when the governing condition is the controlling one.
- Freezing a design before the constructability and access review that would have changed it.
- Ignoring constructability: a design that cannot be built safely is not a completed design.
- Omitting the safety check because the strength check passed.
- Referencing figures, tables, or sources that never appear in the reference list.
Section L
Industry case study
Documented failure related to design speed
A constructed civil works project where this module's decision was made incorrectly or skipped.
Official findings
- Published investigation identified a breakdown between analysis assumption and constructed condition.
Field observations
- The controlling assumption was documented nowhere in the design record.
- No independent check existed at the stage where the error entered the work.
Engineering interpretation
- Interpretation below is student analysis for instructional purposes, not an official finding.
- Map the failure to a step in your own workflow and state where your process would have caught it.
Lessons learned
- Document the assumption, then have someone else check it before it becomes construction.
Source: Summarize the published investigation; cite it in your reference list. Do not reproduce copyrighted report text.
Section M
FE Civil exam connection
Handbook FE Reference Handbook — transportation engineering section (record the section number from your handbook edition).
Exam topics
Handbook formulas
- e+f = V²/(15R)
Weak results here feed your FE Civil Academy weak-area queue for targeted practice.
Question 1 of 2
Score: 0/2In design speed, which item must be established BEFORE the analysis is run?
Section N
Apply it to your project — Design Speed
Complete this using your own capstone project data. Every field is saved to your project record and routed to your advisor with this module's submission.
Inputs and sources
Every value needs a traceable source.
| Quantity | Value | Unit | Source / record |
|---|
Assumptions and consequences
| Assumption | Basis | Consequence if wrong |
|---|
Self-check before submission
Section O
Design challenge
Consulting challenge — Design Speed
Your firm has been retained to deliver the design speed scope for a municipal client on a compressed schedule. Produce the technical position your firm would defend at a public meeting.
Client request: The client wants a defensible recommendation, the basis of design, and an honest statement of what remains unresolved.
Constraints
- Adopted local code edition governs; no exceptions without written variance.
- Budget and schedule are fixed; scope changes require change control.
- Public safety and accessibility requirements are non-negotiable.
Deliverables
- One-page basis of design
- Supporting calculation extract
- Risk and limitation statement
Evaluation
- Technical correctness
- Standard compliance
- Clarity of engineering judgment
- Honest treatment of uncertainty
Section P
Documentation workspace
Write the report section for this module in the academic editor
Section Q
File uploads
Accepted: PDF, DOCX, XLSX, CSV, PNG, JPG, ZIP
No files uploaded yet.
Section R
Deliverable and advisor review
Design speed selection memo with minimum radius check.
Submissions route to your assigned faculty advisor and are scored independently by faculty and administrator rubrics.
Reflection
What was the hardest engineering judgment in this module, and how did you resolve it?
Section S
ABET outcome mapping
Design speed selection memo with minimum radius check. with advisor review and dual scoring.
Assessment: Faculty rubric score and administrator rubric score on this module's submission.
Rubric: Engineering design · Target: 70% of students at or above 'meets expectations'.
Design speed selection memo with minimum radius check. with advisor review and dual scoring.
Assessment: Faculty rubric score and administrator rubric score on this module's submission.
Rubric: Engineering design · Target: 70% of students at or above 'meets expectations'.
Section T
References and further study
AASHTO Green Book (7th Ed.)
Adopted reference — cite section numbers, do not reproduce text.
Design Speed — instructor design procedure
Course template for the calculation package format expected in the final report appendix.
NCEES FE Reference Handbook
Locate the equations used here and note the handbook section for exam recall.
Advisor meeting agenda item
Bring the unresolved decision from this module to your next weekly advisor meeting.