Highway design midterm

What is the Green Book's official title?

A Policy on Geometric Design of Highways and Streets

Who publishes the Green Book?

American Association of State Highway and Transportation Officials (AASHTO)

Is the Green Book a standard or a policy?

It's a policy, not a standard. States adopt it into their own design standards.

What is a key critique of the Green Book?

It is sometimes seen as too rigid, especially by multimodal transportation advocates.

Does the Green Book allow flexibility?

Yes, especially regarding lane widths, shoulder design, and context-sensitive solutions.

What is a common issue with intersections and interchanges in urban areas?

Close spacing and surrounding development often cause congestion and safety issues.

What can cause a road to function below its design capacity?

Short left turn lanes, poorly functioning right-turn lanes, and driveways interrupting flow.

What is the role of human factors in the Green Book?

They influence design speeds, reaction time allowances, pedestrian considerations, etc.

How are land use and transportation connected?

They are deeply intertwined; problems in one often cause or worsen problems in the other.

What is a common source of transportation issues?

Land use decisions such as poorly planned development around interchanges.

What is a 15-minute city?

A concept where all essential services are within a 15-minute walk, bike, or transit ride.

How is the Green Book updated?

Based on empirical research, often from multimillion-dollar studies.

What is recommended when reading journal articles?

Start with the abstract, then read results and methodology as needed—not cover to cover.

What are important features of good highway design?

Capacity, safety, cost-effectiveness, structural durability, and minimal disruption.

What trade-off exists in interchange spacing in the US?

More frequent interchanges increase access but can reduce system efficiency and safety.

What are the major phases of project development?

• Long-range planning

• Programming & funding

• Project planning

• Design

• Right-of-way acquisition

• Construction

• Maintenance & operations

What is eminent domain?

The legal process allowing the government to acquire private property for public use.

Why might a community support a full road closure during construction?

It can significantly reduce construction time and cost (e.g., Winston-Salem Business 40).

What is NCDOT's highway prioritization process called?

SPOT (Strategic Prioritization of Transportation)

What is the STIP?

State Transportation Improvement Program – lists funded projects.

What are the two project types in NCDOT prioritization?

Mobility (adding capacity) and Modernization (upgrading existing infrastructure)

How is project scoring divided for statewide projects?

• 70% Data (congestion, safety, cost-benefit, etc.)

• 20% Division Engineers

• 10% MPOs/RPOs

How does scoring differ at the subregional level?

• 30% Data

• 30% Division

• 40% MPOs/RPOs

Words to avoid using or to use deliberately

Tarmac, jargon/acronyms (provide simple definitions), highway/street/road, design vs posted vs operating speeds, number of lanes, (total for the road, not per direction), alternative modes, roundabout, synchronized signals (say coordinates signals), stoplight/red light (say traffic signal), accident (reported crashes or collisions)

What does 'letting' mean in NCDOT project delivery?

Letting refers to the public announcement and bidding phase, where construction firms can bid on a finalized project design.

What are the phases of NCDOT project development?

Phases include initiation, long-range planning, design development, and letting.

Why is understanding project staging important in design?

Because changes late in the process (e.g., after 90% design) can send all disciplines back to revise their work, causing delays.

What are context sensitive solutions in transportation planning?

• An approach involving community input and environmental awareness to ensure projects suit their specific context.

Why might a community reject a proposed transportation project?

Due to lack of community input, poor context sensitivity, or outdated design not meeting current needs.

Why is public input important in transportation projects?

Public input helps identify community needs, avoid conflict, and ensure that the final design serves its users effectively.

What is a design exception?

A formal deviation from standard design criteria, justified by constraints like environmental impact or historical preservation.

What is a design vehicle?

The type of vehicle a road is designed to accommodate, such as fire trucks or school buses.

What is the design year in highway design?

It refers to the future year a road is designed to serve, usually 20–30 years ahead to account for traffic growth.

What role do design controls play in roadway planning?

They are parameters like vehicle size, design year, and speed, often set by policy and used to guide consistent design.

What does it mean if a design is 'vehicle-centric'?

It prioritizes vehicle mobility and speed over pedestrian, bicycle, or multimodal needs.

Why are roadway design manuals important?

They provide consistent, standardized criteria to ensure safe, predictable road designs across jurisdictions.

What role does design speed play in roadway design?

It affects elements like curve radii and sight distances and is typically set 5 mph above the posted speed.

Why is design speed often set above the posted speed limit?

To ensure that road features like curves and sight distance accommodate slightly higher speeds for safety margins.

What is the relationship between speed and crash rate?

For every 1 mph increase in speed, crash rates increase by about 5%; reducing speeds improves safety.

What is the Solomon Curve?

A graph showing that crash risk increases when vehicles travel much slower or faster than the average traffic speed.

What is inferred design speed?

The speed drivers feel is safe based on the road’s geometry, regardless of posted limits.

Why are left turns considered higher risk in road design?

Left turns require crossing oncoming traffic, increasing the chance of conflict and crashes.

What is the risk of using only the 85th percentile speed to set speed limits?

It can lead to continually increasing speed limits and higher crash rates if used without context or safety data.

What is functional classification of roads?

A system categorizing roads by their service, from local roads to arterials and interstates.

What is a collector road?

Collector roads connect local streets to arterials, balancing access and mobility.

What is access control in roadway design?

It determines how much access is allowed to adjacent properties, from full control (interstates) to no control (neighborhoods).

What is curve radius and why does it matter?

It determines the sharpness of a curve; tighter curves require lower design speeds for safety.

What are horizontal and vertical alignment in road design?

They define the geometric path of a roadway and affect visibility, safety, and drainage.

What is grade in roadway design?

Grade refers to the slope of the road surface, usually expressed as a percentage, affecting vehicle performance and drainage.

Why is slope important in road design?

Slope affects drainage, safety, and vehicle control, especially on inclines or in poor weather.

Why are shoulders and lane widths standardized?

To maintain consistency and safety across regions and vehicle types, especially on interstates and arterials.

What is shoulder width and why is it important in design?

Shoulder width provides space for emergencies, breakdowns, and lateral support; it varies based on roadway type and volume.

How does lane width affect driver behavior?

Wider lanes may increase speed and reduce lane discipline; too narrow lanes increase crash risk.

What is off-tracking and how does it influence design?

Off-tracking occurs when the rear wheels of a long vehicle follow a tighter path than the front, impacting design of curves and roundabouts.

What is the Highway Safety Manual used for?

To provide tools and methods for quantifying the safety effects of roadway design and operational decisions.

What does the CMF Clearinghouse provide?

Crash Modification Factors to estimate safety impacts of different design features or changes.

What is the Highway Capacity Manual used for?

It provides methods to evaluate traffic operations, such as level of service, for various roadway facilities.

What is level of service (LOS) in traffic operations?

A grading system (A–F) representing the quality of traffic flow, with A as free flow and F as stop-and-go congestion.

What is the function of truck aprons in roundabouts?

They provide extra space for large trucks to off-track safely without leaving the paved surface.

What is the benefit of using AutoTurn in roadway design?

It simulates vehicle turning paths to help check for adequate clearance and minimize off-tracking issues.

Why are autonomous and connected vehicles important for future roadway design?

Autonomous vehicles prioritize safety; connected vehicles improve efficiency through communication with each other and infrastructure.

Why are backing-related parking fatalities often more severe?

They often occur in driveways involving children or people in the driveway.

What does the crash data show about pulling in versus backing into parking spaces?

More crashes occur when pulling into spaces than backing in.

Why might backing into a parking space be safer?

you back in when the space is clear, reducing risk during exit.

Why did NC State implement a pull-in parking policy?

To scan license plates since NC only has rear plates.

What was the impact of NC State’s policy on back-in parking frequency?

It dropped from 23.6% to 7.6%.

What limitation does campus-based parking research have?

It's based on younger, more risk-tolerant drivers.

What is a key limitation of crash data in research?

Many minor crashes go unreported.

How can policies unintentionally affect safety?

Enforcement policies (e.g., scanning plates) may reduce crash rates indirectly.

What is a design control in roadway engineering?

A parameter like lane width or design speed that must be justified if changed.

What are examples of design controls?

Lane width, shoulder width, design speed, super elevation, sight distance.

What should be included in a design element table?

Chosen values, sources, and justification.

Why is engineering judgment important in design?

Many problems have multiple valid solutions; trade-offs are required.

How do human factors affect road safety?

Driver expectations influence behavior—unexpected curves raise crash risk.

What is meant by “momentum grades” for trucks?

Downhill speed helps trucks climb the next hill without slowing too much.

Why are shoulders important in roadway design?

Improve safety, drainage, and protect pavement integrity.

What is the typical max super elevation in NC?

8% (6% or less in snowy/icy regions).

What does the crash modification factor (CMF) represent?

A multiplier showing how a design change affects crash risk.

What happens with high super elevation in icy areas?

Vehicles may slide inward on curves with low friction.

Why not design with overly precise super elevation values?

Construction tolerances make such precision impractical.

What is the purpose of a reverse crown?

Both lanes slope the same way—used in flatter curves.

Why limit shoulder slope with super elevation?

To prevent runoff from the shoulder crossing onto the road.

What is stopping sight distance (SSD)?

The distance a driver needs to see and stop safely.

What two components make up SSD?

Brake reaction distance and braking distance.

What is the assumed brake reaction time in SSD?

2.5 seconds (90th percentile of drivers).

What is the unit conversion in SSD calculations?

1.47 (to convert mph and sec to ft).

What affects visibility in sag vertical curves at night?

Headlights may not illuminate obstacles beyond the curve.

What might block sight distance at crests?

Terrain or bridges blocking view of stopped vehicles.

Why shouldn't SSD values be interpolated from tables?

Field tolerances and practicality mean use only listed values.

What are examples of corridor selection impacts?

Noise, light, farmland disruption, displacement of residents.

What’s a common hazardous waste issue in road expansion?

Old leaking underground tanks at gas stations.

Why are mobile home parks hard to displace?

Finding equivalent accommodations is often difficult.

Why must engineers consider environmental justice?

Lower-income areas may be disproportionately affected.

What is a visual/aesthetic impact in road design?

New roads can obstruct scenic views or alter community character.

What is the most photographed spot on Blue Ridge Parkway?

The viaduct — a scenic example of thoughtful road design.

How should engineers use published research?

As a method/template—not always directly applicable.

Why consider driver behavior in design?

Frequent curves in mountains train drivers to expect more.

What happens when trucks hit low-clearance bridges?

Structural damage, road closures—use sacrificial beams to prevent this.

What purpose do warning signs near low bridges or curves serve?

Alert drivers to sight distance limitations or height restrictions.

What should engineers prioritize when routes cross farmlands?

Avoid splitting farms in a way that ruins land usability.