A widely known useful resource in civil engineering and transportation, it affords complete tips and requirements for freeway design and development. This publication, out there in a digital doc format, serves as an authoritative reference for professionals concerned within the planning, design, and operation of roadways. For example, transportation departments usually seek the advice of it when growing new freeway initiatives or upgrading present infrastructure.
Its significance lies in selling uniformity and security in highway design practices throughout jurisdictions. By adhering to its ideas, engineers can make sure that roadways are designed to accommodate numerous car varieties, site visitors volumes, and person wants. Its historic context is rooted within the want for standardized practices to facilitate interstate commerce and enhance transportation security nationwide, making it a significant instrument for infrastructure growth and upkeep.
The following sections will delve into particular functions of the rules, masking subjects resembling geometric design ideas, intersection design issues, and pavement design methodologies. Moreover, the article will discover how these tips contribute to total transportation effectivity, security, and sustainability.
1. Geometric Design Requirements
Geometric design requirements, as outlined throughout the publication, are basic to the security and operational effectivity of roadways. These requirements dictate the bodily format of a highway, influencing elements resembling sight distance, curve radii, and lane widths. Adherence to those requirements is important for creating roads which are protected, comfy, and environment friendly for all customers.
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Horizontal Alignment
Horizontal alignment defines the trail of the highway within the horizontal airplane, encompassing tangents (straight sections) and curves. The collection of applicable curve radii is vital to making sure drivers can safely navigate the highway on the design pace. Inadequate radii can result in elevated crash threat, notably for bigger automobiles. The doc dictates minimal radii primarily based on design pace, superelevation (banking), and facet friction elements.
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Vertical Alignment
Vertical alignment refers back to the profile of the highway, together with grades (slopes) and vertical curves. Steep grades can impede the efficiency of heavy automobiles, resulting in lowered speeds and elevated gas consumption. Vertical curves, used to transition between completely different grades, should be designed to supply ample sight distance. The doc specifies minimal lengths for vertical curves primarily based on design pace and algebraic distinction in grades.
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Cross-Part Parts
Cross-section parts embody lane widths, shoulder widths, and roadside options. Ample lane widths are important for accommodating automobiles of various sizes and offering adequate lateral clearance. Shoulders present area for disabled automobiles, emergency entry, and lateral assist for the pavement construction. Clear roadside zones, freed from obstacles, decrease the severity of crashes if a car leaves the roadway. Particular dimensions for these parts are detailed to reinforce security and operational effectivity.
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Intersection Design
Intersections signify vital factors within the highway community, the place completely different site visitors streams converge and diverge. The doc offers detailed steerage on intersection design, together with sight distance necessities, channelization, and sign timing. Correct intersection design minimizes conflicts between automobiles, pedestrians, and cyclists, decreasing the chance of crashes and bettering site visitors stream. The design additionally focuses on offering ample turning radii and protected crossing choices.
The geometric design requirements throughout the doc function a complete framework for designing protected and environment friendly roadways. By adhering to those requirements, engineers can create roads that meet the wants of all customers whereas minimizing the chance of crashes and congestion. The correct utility of those ideas is essential for guaranteeing the long-term efficiency and sustainability of the transportation community.
2. Freeway Security Rules
Freeway Security Rules are intrinsically linked to the rules introduced within the doc. The useful resource acts as a sensible utility of those ideas, offering concrete methodologies and requirements to attain safer highway design and operation.
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Minimizing Threat
This precept prioritizes decreasing the chance and severity of crashes. The doc immediately addresses this by geometric design requirements that guarantee ample sight distances, applicable curve radii, and clear roadside restoration areas. For instance, specifying minimal stopping sight distance standards at horizontal curves helps forestall rear-end collisions by giving drivers adequate time to react to surprising hazards. This proactive strategy mitigates potential risks inherent in highway journey.
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Forgiving Roadside Design
The idea of a forgiving roadside design acknowledges that driver errors can happen. The doc offers requirements for clear zones, breakaway helps for indicators and lighting, and the strategic placement of guardrails to attenuate the affect of off-road excursions. Correctly designed clear zones, freed from inflexible obstacles, enable errant automobiles to soundly get better. Breakaway helps cut back the severity of collisions with roadside appurtenances. These measures cut back the potential for severe accidents and fatalities.
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Site visitors Calming Measures
The doc offers steerage on incorporating site visitors calming methods to scale back car speeds and enhance pedestrian security, notably in city or residential areas. These measures embody pace humps, raised crosswalks, and curb extensions. Decrease speeds present drivers with extra time to react to surprising conditions, growing security for all highway customers. The rules promote an atmosphere that’s extra accessible to pedestrians and cyclists.
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Separation of Site visitors
Separating conflicting site visitors streams is a core freeway security precept. The doc illustrates this by requirements for medians, barrier methods, and grade separations. Medians forestall head-on collisions by bodily separating opposing lanes of site visitors. Barrier methods, resembling concrete limitations, redirect errant automobiles and stop them from crossing into opposing lanes. Grade separations, resembling overpasses and underpasses, get rid of at-grade crossings of roadways or railways, considerably decreasing crash threat.
The Freeway Security Rules present the overarching philosophy that guides the detailed specs within the doc. By incorporating these ideas into roadway design, engineers can create safer and extra forgiving transportation methods. The adherence to those tips interprets into tangible enhancements in highway security, in the end saving lives and decreasing the societal prices related to crashes. They permit to supply the doc that’s broadly used as greatest practices by governments all over the world.
3. Capability Evaluation Strategies
Capability Evaluation Strategies, as knowledgeable by the ideas detailed on this useful resource, present a scientific strategy to evaluating the operational efficiency of transportation services. These strategies are vital for understanding how effectively a highway or intersection handles site visitors quantity, and for figuring out potential bottlenecks or areas of congestion. The doc’s affect on these strategies ensures a standardized and dependable strategy to site visitors evaluation.
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Degree of Service (LOS) Dedication
The publication defines Degree of Service (LOS) standards, that are qualitative measures describing site visitors operational circumstances. These standards, starting from LOS A (free stream) to LOS F (compelled stream), are used to evaluate the standard of site visitors stream on a given roadway phase. Capability evaluation strategies, guided by the doc, allow engineers to find out the LOS for present or deliberate roadways, offering a foundation for figuring out areas the place enhancements are wanted. For instance, an evaluation of a freeway phase revealing an LOS of D might immediate suggestions for widening the highway or implementing ramp metering to enhance site visitors stream.
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Freeway Capability Guide (HCM) Procedures
The HCM, carefully aligned with the requirements in query, offers particular procedures for analyzing the capability and efficiency of varied transportation services, together with freeways, arterials, and intersections. These procedures incorporate elements resembling site visitors quantity, pace, density, and geometric traits to estimate the capability and LOS of a facility. The doc serves as a foundational doc for the HCM methodologies, guaranteeing that analyses are performed in a constant and dependable method. For example, the HCM procedures for signalized intersections, underpinned by this useful resource, are used to optimize sign timing and enhance intersection effectivity.
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Site visitors Demand Forecasting
Correct site visitors demand forecasting is important for efficient capability evaluation. These strategies, knowledgeable by the doc’s ideas, are used to foretell future site visitors volumes primarily based on elements resembling inhabitants development, financial growth, and land use adjustments. Site visitors demand forecasts are used to evaluate the adequacy of present transportation infrastructure and to plan for future enhancements. For example, transportation companies use site visitors forecasting fashions, aligned with the doc’s tips, to find out the necessity for brand new freeway development or transit enlargement initiatives.
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Simulation Modeling
Simulation modeling, usually employed along side analytical strategies, offers a strong instrument for evaluating the efficiency of advanced transportation methods. Simulation fashions, developed in accordance with the doc’s tips, can simulate site visitors stream below varied situations, permitting engineers to evaluate the affect of various design alternate options or site visitors administration methods. For instance, a simulation mannequin could also be used to guage the effectiveness of a proposed roundabout at a busy intersection, accounting for elements resembling car speeds, turning actions, and pedestrian volumes.
In conclusion, Capability Evaluation Strategies, as knowledgeable by the rules throughout the doc, supply a complete framework for evaluating the efficiency of transportation services. These strategies allow engineers to determine potential bottlenecks, optimize site visitors stream, and plan for future enhancements to the transportation community. The standardized strategy promoted by the doc ensures that capability analyses are performed persistently throughout jurisdictions, facilitating knowledgeable decision-making and environment friendly allocation of transportation sources.
4. Intersection Design Tips
Intersection design tips, as detailed throughout the useful resource, are a vital part of roadway engineering, immediately influencing security, effectivity, and total site visitors stream. The doc offers a complete framework for designing intersections that accommodate numerous person wants whereas minimizing conflicts and potential hazards.
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Sight Distance Necessities
The publication establishes minimal sight distance standards for intersections, guaranteeing that drivers have ample visibility of approaching automobiles and pedestrians. These necessities fluctuate primarily based on design pace, intersection geometry, and the presence of site visitors management units. The doc dictates particular sight triangles that should be maintained at intersections to supply drivers with adequate time to react to potential hazards. Failure to fulfill these sight distance necessities can considerably improve the chance of crashes, notably at unsignalized intersections.
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Intersection Geometry
The doc offers detailed steerage on intersection geometry, together with the design of turning lanes, channelization, and median islands. Correct intersection geometry is important for guiding site visitors stream, decreasing conflicts, and bettering total intersection effectivity. For instance, the doc specifies minimal radii for turning lanes to accommodate completely different car varieties, guaranteeing that vans and buses can safely navigate the intersection. Channelization, by using raised islands or pavement markings, helps to separate conflicting site visitors actions, decreasing the probability of collisions.
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Site visitors Management Gadgets
The rules tackle the choice and placement of site visitors management units at intersections, together with site visitors alerts, cease indicators, and yield indicators. The doc offers standards for warranting site visitors alerts primarily based on elements resembling site visitors quantity, pedestrian quantity, and crash historical past. Sign timing, lane configurations are optimized primarily based on site visitors research. Moreover, the rules specify requirements for the scale, form, and placement of site visitors indicators to make sure that they’re clearly seen to drivers.
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Pedestrian and Bicycle Lodging
The rules emphasize the significance of accommodating pedestrians and bicyclists at intersections. The doc offers requirements for crosswalk placement, pedestrian sign timing, and bicycle lane design. Accessibility tips are included into intersection design to make sure that people with disabilities can safely navigate the intersection. Curb ramps, detectable warnings, and accessible pedestrian alerts are important parts of an accessible intersection design.
In summation, the intersection design tips on this doc make sure that intersections are designed to be protected, environment friendly, and accessible for all customers. Adherence to those tips is important for minimizing conflicts, decreasing crashes, and bettering the general efficiency of the transportation community. The applying of those requirements requires cautious consideration of site visitors volumes, pedestrian wants, and site-specific circumstances to attain optimum intersection design.
5. Pavement Design Standards
Pavement design standards are essentially linked to the rules set forth within the digital doc, serving as a sensible utility of its broader ideas. These standards dictate the structural design of roadways, guaranteeing they’ll stand up to anticipated site visitors hundreds, environmental circumstances, and time-related deterioration. The doc’s relevance stems from its provision of standardized methodologies and materials specs that immediately affect pavement design selections. Its significance is underscored by its function in offering a constant strategy to pavement engineering, resulting in extra sturdy and cost-effective roadways.
The American Affiliation of State Freeway and Transportation Officers (AASHTO) Information for Design of Pavement Constructions, generally identified in its digital format, offers engineers with instruments for calculating the required pavement thickness and materials properties primarily based on elements resembling site visitors quantity, axle hundreds, subgrade soil traits, and local weather. These calculation methodologies tackle asphalt pavements and concrete pavements together with inflexible pavement design or versatile pavement design. For instance, a freeway mission in a area with heavy truck site visitors and freeze-thaw cycles necessitates a pavement construction designed in accordance with these tips to forestall untimely failure. The rules make sure that pavement supplies can stand up to the stresses of steady use, decreasing upkeep prices and guaranteeing long-term roadway efficiency. Sensible utility might contain life cycle price evaluation. This offers companies with an optimum plan to attenuate prices of roadway networks.
The connection between the doc and pavement design is essential for guaranteeing uniformity and reliability in freeway infrastructure. By adhering to those established standards, engineers can design pavements that meet particular efficiency necessities, decreasing the chance of untimely failures and minimizing long-term upkeep prices. This, in flip, results in safer and extra environment friendly transportation methods. It permits long run lifetime of infrastructure.
6. Drainage Design Concerns
Drainage design issues are integral to the great tips contained throughout the useful resource. The correct administration of stormwater runoff is important for preserving the structural integrity of roadways, sustaining protected driving circumstances, and minimizing environmental impacts. With out ample drainage, roadways are vulnerable to water injury, together with pavement cracking, base erosion, and slope instability. The digital useful resource offers particular methodologies and requirements for designing drainage methods that successfully seize and convey stormwater, stopping injury to the transportation infrastructure.
The design issues detailed throughout the digital useful resource embody varied facets of drainage system design, together with the collection of applicable drainage constructions, the calculation of runoff charges, and the analysis of hydraulic efficiency. For instance, the doc offers steerage on figuring out the suitable dimension and spacing of culverts primarily based on watershed traits and anticipated rainfall occasions. It additionally addresses the design of stormwater detention basins to regulate peak stream charges and mitigate flooding. Correct drainage design ensures that roadways stay protected and serviceable in periods of heavy rainfall. Poor drainage can result in hydroplaning, lowered visibility, and elevated crash threat. Efficient drainage additionally protects the encircling atmosphere by stopping erosion and sedimentation, decreasing the chance of water air pollution.
The particular methodologies and requirements detailed throughout the doc mirror a dedication to sustainable drainage practices. These requirements encourage using inexperienced infrastructure methods, resembling bioswales and permeable pavements, to advertise infiltration and cut back runoff volumes. By incorporating these methods into drainage design, engineers can decrease the environmental impacts of roadways and improve the general sustainability of the transportation system. Drainage design issues, subsequently, are usually not merely an addendum however a significant and built-in part of the useful resource, guaranteeing the long-term sturdiness, security, and environmental duty of transportation infrastructure.
7. Site visitors Management Gadgets
The implementation and standardization of site visitors management units are intrinsically linked to the ideas and proposals discovered throughout the digital doc. These units, encompassing indicators, alerts, and pavement markings, are essential for speaking rules, warnings, and steerage to highway customers, thereby selling security and effectivity. The doc serves as a key reference for guaranteeing constant utility and optimum effectiveness of those units throughout varied roadway environments.
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Standardization of Design and Utility
The digital doc promotes standardization within the design and utility of site visitors management units, aligning with the Guide on Uniform Site visitors Management Gadgets (MUTCD). Constant designs make sure that highway customers can readily perceive the supposed message no matter location. For example, standardized warning signal shapes and colours convey particular hazards, resembling curves or intersections, permitting drivers to anticipate and react appropriately. The doc additionally offers steerage on the suitable placement and spacing of those units to maximise visibility and effectiveness.
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Warrants for Site visitors Sign Set up
The useful resource informs warrants, or justifications, for putting in site visitors alerts at intersections primarily based on elements like site visitors quantity, pedestrian exercise, and crash historical past. It outlines particular thresholds that should be met earlier than a sign is deemed needed. For instance, if an intersection experiences a excessive quantity of left-turning automobiles or pedestrian crossings that exceed a sure threshold, a site visitors sign could also be warranted to enhance security and cut back delays. These warrants make sure that site visitors alerts are put in solely when justified by site visitors circumstances, avoiding pointless delays and disruptions.
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Pavement Marking Requirements
The requirements embody detailed specs for pavement markings, together with lane traces, edge traces, and crosswalks. The doc specifies the size, colours, and retroreflectivity necessities for these markings. Correctly maintained pavement markings improve visibility, particularly throughout nighttime or antagonistic climate circumstances, guiding drivers and pedestrians and bettering security. For instance, the width and spacing of lane traces are essential for sustaining lane self-discipline and stopping lane departures.
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Signal Placement and Visibility
Optimum signal placement is vital for guaranteeing that drivers can readily see and perceive site visitors management indicators. The digital doc offers steerage on signal peak, lateral offset, and viewing angles to maximise signal visibility. It additionally addresses the significance of minimizing obstructions, resembling bushes or utility poles, that may obscure indicators. Correct signal placement ensures that drivers have ample time to react to the data conveyed by the signal, decreasing the chance of crashes. For instance, advance warning indicators for upcoming intersections or curves should be positioned at a adequate distance to permit drivers to regulate their pace and place safely.
The connection between site visitors management units and the useful resource highlights the significance of standardized, data-driven approaches to roadway security. By adhering to those tips, transportation companies can make sure that site visitors management units are deployed successfully to scale back crashes, enhance site visitors stream, and improve the general security and effectivity of the transportation community. Constant utility of those ideas is important for creating predictable and protected driving environments throughout completely different jurisdictions.
8. Sustainable Practices Integration
Sustainable practices integration represents a rising crucial inside transportation engineering, influencing how roadways are deliberate, designed, constructed, and maintained. Whereas not explicitly the core focus, the ideas throughout the digital useful resource more and more mirror issues for environmental stewardship, useful resource conservation, and long-term lifecycle impacts.
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Life Cycle Price Evaluation (LCCA)
Life Cycle Price Evaluation (LCCA) is a technique for evaluating the whole price of a mission over its total lifespan, together with preliminary development, upkeep, rehabilitation, and eventual disposal. The useful resource encourages LCCA to tell materials choice and design selections, favoring choices with decrease long-term environmental impacts. For instance, deciding on a pavement materials with an extended service life or decrease embodied vitality can lead to important price financial savings and lowered carbon footprint over the pavement’s life cycle. This aligns with sustainable practices by selling useful resource effectivity and minimizing environmental burdens.
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Inexperienced Infrastructure and Stormwater Administration
Conventional stormwater administration practices usually depend on standard drainage methods that channel runoff immediately into waterways, probably resulting in erosion and air pollution. The useful resource promotes the mixing of inexperienced infrastructure methods, resembling bioswales, permeable pavements, and rainwater harvesting, to handle stormwater runoff in a extra sustainable method. These methods cut back runoff quantity, enhance water high quality, and improve habitat worth. Implementing inexperienced infrastructure practices can contribute to extra resilient and environmentally accountable transportation methods.
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Materials Choice and Recycling
The collection of development supplies has important environmental implications, contemplating elements resembling embodied vitality, useful resource depletion, and waste technology. The useful resource encourages using recycled supplies, resembling recycled asphalt pavement (RAP) and recycled concrete combination (RCA), to scale back reliance on virgin supplies and decrease landfill waste. These practices contribute to a round economic system by diverting waste streams and conserving pure sources. Additional, the selection of regionally sourced supplies reduces transportation-related emissions, contributing to a smaller carbon footprint.
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Context Delicate Options (CSS)
Context Delicate Options (CSS) contain designing transportation initiatives which are delicate to the encircling atmosphere and neighborhood context. This strategy considers not solely the practical necessities of the roadway but additionally the aesthetic, social, and environmental impacts of the mission. The useful resource implicitly helps CSS by encouraging a holistic strategy to transportation planning and design that considers a broad vary of stakeholder values and neighborhood wants. CSS can result in extra sustainable outcomes by minimizing environmental impacts, enhancing neighborhood livability, and selling multimodal transportation choices.
The mixing of sustainable practices throughout the framework of the digital useful resource represents a shift in the direction of a extra environmentally acutely aware strategy to transportation engineering. By incorporating LCCA, inexperienced infrastructure, materials recycling, and CSS, transportation companies can create roadways that aren’t solely protected and environment friendly but additionally environmentally accountable and socially helpful. The continuing evolution of this doc will seemingly additional emphasize these facets, solidifying the function of sustainable practices in shaping the way forward for transportation infrastructure.
Incessantly Requested Questions
The next questions tackle frequent inquiries and misconceptions surrounding the use and interpretation of the publication, a key useful resource for transportation professionals.
Query 1: What’s the major function of the Information for Design of Pavement Constructions?
The first function is to supply complete tips and methodologies for the structural design of pavements. It goals to make sure that roadways are designed to face up to anticipated site visitors hundreds and environmental circumstances, selling long-term efficiency and minimizing upkeep prices.
Query 2: Is the digital format a suitable substitute for the bodily printed model?
The digital format offers the identical content material because the printed model and affords advantages resembling searchability and portability. The digital copy is taken into account a suitable substitute, offered that the person ensures the model is present and genuine.
Query 3: How incessantly is the publication up to date, and why are updates needed?
Updates are sometimes launched periodically to mirror developments in pavement expertise, materials science, and design methodologies. Updates are needed to make sure that pavement designs are primarily based on the most recent analysis and greatest practices, resulting in extra sturdy and cost-effective roadways.
Query 4: Are the rules obligatory for all transportation initiatives?
Adoption of the rules varies by jurisdiction. Whereas it serves as an authoritative useful resource, state and native companies might have their very own particular design requirements and rules. Nevertheless, the ideas throughout the publication usually inform these native requirements.
Query 5: Does the publication tackle the design of each versatile and inflexible pavements?
The publication covers each versatile (asphalt) and inflexible (concrete) pavement design methodologies, offering steerage on materials choice, layer thickness dedication, and efficiency prediction for each pavement varieties. It comprises formulation for inflexible and versatile pavement.
Query 6: How does it contribute to sustainable transportation infrastructure?
The doc promotes sustainable practices by using Life Cycle Price Evaluation, encouraging the collection of supplies and designs that decrease environmental impacts and maximize long-term efficiency. It additionally helps using recycled supplies and inexperienced infrastructure methods.
In abstract, the Information for Design of Pavement Constructions is a vital useful resource for transportation professionals looking for to design sturdy, cost-effective, and sustainable pavements. Understanding the doc’s function, updates, and utility is vital for guaranteeing the long-term efficiency of the transportation infrastructure.
The following part will discover future traits and challenges associated to the doc’s utility within the evolving discipline of transportation engineering.
Sensible Purposes of Pavement Design Requirements
This part offers focused recommendation on leveraging the insights contained throughout the digital doc to reinforce roadway design and infrastructure administration practices. These tips supply strategic approaches for optimizing using this necessary useful resource.
Tip 1: Prioritize Life Cycle Price Evaluation. Combine Life Cycle Price Evaluation (LCCA) into the pavement design course of. This methodology permits for the analysis of whole mission prices, together with preliminary development, upkeep, and rehabilitation, over the whole lifespan of the pavement. LCCA informs decision-making by figuring out cost-effective and sustainable design alternate options.
Tip 2: Seek the advice of the Newest Version. All the time reference essentially the most present version of the publication to make sure that design practices align with the newest analysis and technological developments. Updates sometimes incorporate refinements in design methodologies, materials specs, and efficiency prediction fashions.
Tip 3: Adapt to Native Circumstances. Whereas the doc offers complete tips, pavement design should be tailor-made to particular native circumstances, together with local weather, site visitors patterns, soil traits, and materials availability. Customizing design parameters to account for regional variations improves pavement efficiency and sturdiness.
Tip 4: Combine Sustainable Supplies. Promote using sustainable supplies, resembling recycled asphalt pavement (RAP) and recycled concrete combination (RCA), in pavement development. These supplies cut back reliance on virgin sources, decrease landfill waste, and decrease the embodied vitality of pavements.
Tip 5: Take into account Drainage Design. Incorporate efficient drainage design into pavement development to mitigate the detrimental results of water infiltration. Correct drainage methods forestall pavement cracking, base erosion, and subgrade weakening, extending pavement life and enhancing security.
Tip 6: Implement Efficiency Monitoring. Set up a strong pavement efficiency monitoring program to trace pavement situation, determine deterioration traits, and inform upkeep and rehabilitation selections. This data-driven strategy permits proactive administration of pavement property, minimizing lifecycle prices and maximizing pavement service life.
Tip 7: Prioritize Person Accessibility. Designs should incorporate accessibility requirements and tackle the wants of numerous customers. This consists of ample signage, easy transitions, and protected crossings.
The following tips underscore the significance of a complete and adaptable strategy to pavement design. Integrating LCCA, native circumstances, sustainable supplies, drainage design, and efficiency monitoring permits transportation companies to create extra sturdy, cost-effective, and environmentally accountable roadways.
The conclusion will present a abstract of the details and spotlight future traits associated to the useful resource and its utility in transportation engineering.
Conclusion
This exposition has detailed the importance of the “inexperienced ebook aashto pdf” as a foundational useful resource in transportation engineering. Its tips for geometric design, security, capability evaluation, intersection design, pavement design, drainage, site visitors management, and sustainable practices collectively contribute to safer, extra environment friendly, and environmentally accountable roadways. The doc’s affect extends throughout varied aspects of transportation infrastructure growth and upkeep.
Shifting ahead, continued adherence to and evolution of the ideas outlined within the “inexperienced ebook aashto pdf” are important for addressing the challenges of rising site visitors volumes, growing older infrastructure, and growing calls for for sustainability. The continuing utility of its tips shouldn’t be merely a technical train, however a dedication to making sure the security and high quality of life inside communities served by transportation networks.
