Roundabouts
Evaluate and design a modern roundabout as an intersection control alternative.
Section progress
0% of the workflow complete
Highway & Traffic Engineering · Geometric design, traffic analysis, pavement, intersections and roadway safety to AASHTO and MUTCD.
Deliverable: Roundabout feasibility and geometric design package with capacity analysis.
Minimum tables, figures and equations for Roundabouts
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 Roundabouts lecture and the worked example so you know what "Roundabout feasibility and geometric design package with capacity analysis." 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.
Roundabouts
Evaluate and design a modern roundabout as an intersection control alternative.
Section B
Engineering story
A real project situation that frames this module
It is week 5 of implementation and the transportation engineering team has reached roundabouts. Evaluate and design a modern roundabout as an intersection control alternative. The DOT design reviewer asks one question: what establishes that roundabout geometry?
Setting inscribed diameter too small for the design vehicle's turning path. Because capacity analysis via gap-acceptance / HCM roundabout methodology, 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 truck apron design for large design vehicles, 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 roundabout geometry, and is that record in the project data inventory?
- Does HCM (7th Ed.), Ch. 22, govern here — and is that the edition adopted by the jurisdiction?
- What is the acceptance criterion for capacity analysis via gap-acceptance / HCM roundabout methodology, and was it written before the result was known?
- Is the documented procedure valid for the conditions this project actually presents?
- 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 roundabouts cites HCM (7th Ed.), Ch. 22, and shows the record behind each input. Your roundabout feasibility and geometric design package with capacity analysis. is reviewed the same way — traceability is assessed before arithmetic.
Technical
Roundabout geometry controls the numbers this module hands forward. Capacity analysis via gap-acceptance / HCM roundabout methodology determines whether those numbers remain valid once conditions change.
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 comparative safety performance vs, 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 roundabout geometry; 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
Reducing conflict points from 32 (signalized) to 8 (roundabout) is a geometric, not just operational, safety improvement. Everyone who walks, rides or drives the corridor every day live with that outcome long after the semester ends.
Section D
Learning objectives
By the end of this module you will be able to:
- 1.Justify roundabout geometry, using this project's own conditions rather than a textbook case.
- 2.Explain capacity analysis via gap-acceptance / HCM roundabout methodology, using this project's own conditions rather than a textbook case.
- 3.Apply truck apron design for large design vehicles, using this project's own conditions rather than a textbook case.
- 4.Compare pedestrian and bicycle accommodation (splitter island refuge, ramps), using this project's own conditions rather than a textbook case.
- 5.Apply HCM (7th Ed.), Ch. 22, and cite the section that governs your acceptance decision.
- 6.Produce roundabout feasibility and geometric design package with capacity analysis. 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
The engineering content of roundabouts
Evaluate and design a modern roundabout as an intersection control alternative. That single sentence hides the substance of the module: roundabout geometry, and capacity analysis via gap-acceptance / HCM roundabout methodology. 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. Truck apron design for large design vehicles — 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.
- Roundabout geometry: inscribed circle diameter, entry/exit radii, deflection
- Capacity analysis via gap-acceptance / HCM roundabout methodology
- Truck apron design for large design vehicles
- Pedestrian and bicycle accommodation (splitter island refuge, ramps)
- Comparative safety performance vs. signalized intersections (conflict point reduction)
Photo 1. The engineering content of roundabouts in practice — Peak-hour demand: the flow rate that drives capacity, delay and level-of-service analysis.
Wikimedia Commons, CC BY 2.0
Decision logic: the procedure that replaces a closed-form solution
Roundabouts is governed by a documented procedure rather than a single expression, so the decision logic is the deliverable: what you accept, what you reject, and on what evidence. Roundabout geometry.
Write the acceptance criterion before you look at the result. Capacity analysis via gap-acceptance / HCM roundabout methodology — recording the criterion afterwards lets it be shaped to fit the number you happened to get.
Photo 2. Decision logic: the procedure that replaces a closed-form solution 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 roundabouts
HCM (7th Ed.), Ch. 22, governs this module: Roundabout capacity and LOS AASHTO Green Book (7th Ed.), Ch. 9, adds the second constraint: Roundabout geometric design
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 comparative safety performance vs together with an independent check by someone who did not perform the work.
- Controlling criterion for this module: roundabout geometry.
- Adopted reference: HCM (7th Ed.) — cite Ch. 22 by number.
- Failure mode guarded: a geometric or control element that puts drivers in a conflict they cannot resolve.
- Evidence produced: Roundabout feasibility and geometric design package with capacity analysis..
Photo 3. Constraints, adopted standards and the safety case for roundabouts 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
Every method has a domain of validity. State the range of geometry, loading, material behaviour or flow regime over which your approach holds, and state what you would do instead beyond it.
For this project, the boundary you are most likely to push is comparative safety performance vs. 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 — roundabout geometry; capacity analysis via gap-acceptance / HCM roundabout methodology — each with a unit and a source record.
- 2. Confirm HCM (7th Ed.) is the adopted edition and locate Ch. 22.
- 3. State the assumptions and the acceptance criterion for roundabout geometry.
- 4. Execute the documented procedure, recording each judgement and the evidence behind it.
- 5. Test the result against truck apron design for large design vehicles.
- 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 roundabout feasibility and geometric design package with capacity analysis. and submit it to the DOT design reviewer for review.
Decision points
- Is every input behind roundabout geometry traceable? If not — stop and collect the record.
- Does the result satisfy capacity analysis via gap-acceptance / HCM roundabout methodology? 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 — setting inscribed diameter too small for the design vehicle's turning path?
Quality checklist
- Documented: roundabout geometry
- Documented: capacity analysis via gap-acceptance / HCM roundabout methodology
- Documented: truck apron design for large design vehicles
- HCM Ch. 22 cited by section number
- Procedure steps recorded in order with evidence
- Acceptance criterion recorded before the result
- Independent check signed and dated
- Roundabout feasibility and geometric design package with capacity analysis. attached and named per the course convention
Section H
Interactive visualization
Roundabouts — step-through
Advance one frame at a time. Each frame adds one engineering decision to the previous state.
Step 1 of 6
Screen intersection for roundabout suitability
Section I
Applicable codes and standards
HCM
7th Ed. · Ch. 22
Adopted design/analysis reference governing this module.
Relevance: Roundabout capacity and LOS
Reference the section number and edition in your calculation package. Do not reproduce code text.
AASHTO Green Book
7th Ed. · Ch. 9
Adopted design/analysis reference governing this module.
Relevance: Roundabout geometric design
Reference the section number and edition in your calculation package. Do not reproduce code text.
FHWA
Current · Roundabouts: An Informational Guide
Adopted design/analysis reference governing this module.
Relevance: Roundabout design practice
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
- Setting inscribed diameter too small for the design vehicle's turning path
- Ignoring pedestrian crossing accommodation at splitter islands
- Applying signalized-intersection LOS thresholds directly to roundabout analysis
- Treating roundabout geometry as a given instead of establishing it from a project record.
- Producing roundabout feasibility and geometric design package with capacity analysis. without showing how capacity analysis via gap-acceptance / HCM roundabout methodology was satisfied.
- Recording the outcome of this module without recording the judgement and evidence that produced it.
- Missing comparative safety performance vs, 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.
- Leaving boundary conditions undefined so the model is not reproducible by an independent checker.
- Using inputs that no field record, laboratory report, or published source supports.
- Stopping at output and skipping verification — an unverified number is not an engineering result.
Section L
Industry case study
Roundabout Conversion Safety Studies (FHWA)
Multiple U.S. intersections converted from signal/stop control
Official findings
- FHWA-documented conversions show 35–90% reductions in injury crashes after converting to modern roundabouts.
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
- Reducing conflict points from 32 (signalized) to 8 (roundabout) is a geometric, not just operational, safety improvement.
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
Weak results here feed your FE Civil Academy weak-area queue for targeted practice.
Question 1 of 2
Score: 0/2In roundabouts, which item must be established BEFORE the analysis is run?
Section N
Apply it to your project — Roundabouts
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 — Roundabouts
Your firm has been retained to deliver the roundabouts 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, DWG, DXF, Civil3D, VISSIM
No files uploaded yet.
Section R
Deliverable and advisor review
Roundabout feasibility and geometric design package with capacity analysis.
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
Roundabout feasibility and geometric design package with capacity analysis. 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'.
Roundabout feasibility and geometric design package with capacity analysis. 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
HCM (7th Ed.)
Adopted reference — cite section numbers, do not reproduce text.
AASHTO Green Book (7th Ed.)
Adopted reference — cite section numbers, do not reproduce text.
FHWA (Current)
Adopted reference — cite section numbers, do not reproduce text.
Roundabouts — 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.