2.4.2. Balancing Safety, Traffic Service, and Design Exceptions

“CSS consider the function of streets and roads relative to their context in terms of access and mobility for all users.”

ITE, Smart Growth Transportation Guidelines

Community settings require a greater need for flexibility.  An understanding of the functional, operational, and safety impacts of the design criteria and a need to consider alternative methods are necessary to achieve the goals of CSS. 

Balancing Design Criteria and Exceptions with Community Values

To balance transportation needs with community and environment needs, consider the roadway’s functional classification.  The balance will be different on an arterial than on a collector or local street.  Design criteria is based on the functional classification of the roadway, the type of improvement, and design traffic and varies with the adjacent land use and terrain surrounding the project.  Design criteria have been developed based on:

• The safety of all users to the greatest extent practical
• Optimal mobility for all users to reach their destinations as expeditiously as possible
• Cost and availability of funding

 

 

 

 

 

 

 

 

 

 

One controlling geometric design criterion is design speed.  Horizontal and vertical alignment, pavement cross slope, and sight distance depend on design speed.  Selection of design speed depends on functional classification, land use, terrain, and design traffic. Minimum horizontal curvature is another controlling design criterion, which is dependent upon design speed and the maximum pavement cross-slope rate.  Another controlling design element is lane width, which is dependent on functional classification, design traffic, and land use. 

 

Under ideal conditions, the highest design speed, the largest radius curve, widest lanes practical provide for the safe and efficient movement of traffic at the highest speed.  However, there are cases where the highest speed is not desirable and ideal conditions do not always exist.  A design speed should be selected that is consistent with the roadway function, environment, and other features along the roadway.  A lower design speed can provide the opportunity for additional design enhancements.  It is important that project stakeholders agree on their goals for design speed.  Consider the impact of vehicle types and sizes on the design and how this impacts other users.  A grade separated interchange versus an at-grade intersection is another example of an ideal condition. 

 

Table 2.3 provides a general comparison of design criteria and characteristics.  The more conservative design criteria are associated with wider lanes and shoulders and flatter curvature and grades, etc.

 

Table 2.3 Generalized Comparison of Design Criteria and Characteristics
Characteristic	Design Criteria
	More Conservative	Less Conservative
Functional Classification	Arterials	Collectors/Local Streets
	More Through Traffic /Regional Destinations	Less Through Traffic / Regional Destinations
	Less Local Access	More Local Access
Volume	Higher	Lower
Speed	Higher	Lower
Land Use Character	Rural	Urban
Type of Improvement	New Construction	3R and Pavement Rehabilitation
Terrain	Level	Rolling or Mountainous

 

With regard to terrain, it may be prohibitively expensive or physically impossible to attain higher design criteria values for projects located in rolling and mountainous terrain.  Use of high design criteria values may impact roadside development more than the use of less restrictive criteria.  This latter point is particularly important.  Also, context-sensitive solutions can have a significant positive influence in an urban setting or in built up areas, where there is more local traffic and pedestrian activity.

 

As noted in Table 2.4, higher design speeds are consistent with a high volume rural principal arterial in flat terrain, where lower design speeds are consistent with a lower volume local urban street in mountainous terrain.  Obviously, there are projects that fall in between these extremes, with some characteristics favoring the more conservative values and other characteristics reflecting the less restrictive criteria.  Design speeds should be consistent with the expectations of facility users.  The type and quality of enhancements is reflective of the level of roadway design (i.e., certain street enhancements are more appropriate for slower speeds and urban settings, while others are more fitting in a rural setting).

 

Designers should take care not to arbitrarily reduce the mobility of motorized traffic on arterial routes, but should consider and incorporate the mobility of other users.  This point needs to be discussed with project stakeholders.  Table 2.4 also provides a general understanding of the impact of selecting ideal values for the three primary design criteria on community sensitive values. 

 

Table 2.4 Ideal Design Criteria Versus Context-Sensitive Values
Design Criteria	Primary Negative Value	Possible Secondary Negative Value
High Design Speed	May encourage users to travel at higher speeds	Could negatively impact abutting properties
		May reduce comfort and safety for bicyclists and pedestrians
		May impede economic viability of abutting commercial properties, especially those in downtown
Large Radius Curve	Potential for unacceptable impacts to natural or man-made resources
	May result in unnecessary demolition of adjacent properties
	May be in conflict with the existing topography on scenic routes
	May encourage users to travel at higher speeds	Same possible secondary negative values related to higher design speed
Wide Travel Lanes	May encourage users to travel at higher speeds	Same possible secondary negative values related to higher design speed
	May increase the "footprint" of a facility	May negatively impact sensitive abutting environmental resources
		May cause significant right-of-way impacts (impossible or prohibitively expensive to acquire)
	May present difficulty in accommodating non-motorized users (bike lanes) in tight corridors
	May negatively impact parking in commercial areas, particularly downtown

 

Once the basis of the design standards are understood, designers can make appropriate decisions on how to properly apply the standards and this can be communicated to stakeholders.  It is important to identify the context of the project and evaluate how the design criteria will affect it.  Documentation of the recommended guidelines and project specific objectives for critical design features such as design speed, lane and shoulder widths, horizontal and vertical alignment, superelevation, turn lanes, and taper rates should be made.

 

Designers are generally more open to exceptions on routes of lower functional classification.  Collectors and local streets carry lower volumes of travel over shorter distances and typically have more familiar users.  Design exceptions can have a greater impact on high speed arterials.  Design exceptions can be justified based on safety and operational analysis, cost comparisons, stakeholder input, and allow adverse impacts to adjacent resources to be avoided.  Consideration should be given to stakeholder input when taking into account design exceptions.  A CSS uses the design flexibility in the standards and guidelines to incorporate the needs and desires of a community.  This requires using experience, study results, judgment, ingenuity, and creativity to apply design criteria to solve unique problems. 

 

Problems and alternatives must take into account functional classification.  The significant choices that designers make in developing alternatives include design speed, design traffic conditions, and design vehicles. According to the AASHTO Green Book, design speed is used to determine geometric design features and should be logical with respect to topography, adjacent land use, and functional classification.  Design speed is thus an important choice that designers will make.  This choice should be made recognizing the context of the project.