Safe Routes Scoop

Transportation Engineering Design Manuals: What They Do

this peak demand period with other hours of the day to illustrate fluctuations in demand. Many localities use trip and parking generation data when considering applications for new development to determine if adequate parking will be provided and whether to require the developer to pay for such things as a new traffic signal or other measures to handle the estimated increase in traffic expected from the new development. The report is updated every five to ten years and represents the largest available dataset for parking demand.


The existing guidelines for roadway engineering meet widely accepted professional standards and are intended to produce uniform designs that are easily recognizable to drivers and pedestrians. They are not intended, however, to limit the flexibility of roads to serve peculiar local conditions. Safety, environmental impacts, legal concerns, and other local elements are all contributing factors in roadway design. Engineering resources encourage flexibility, and publications such as the AASHTO Green Book are clear in stating that the manual is not meant to prohibit designs that are sensitive to local needs. Rather, the manuals are intended to facilitate effective design by identifying features of safe and accessible roadways. For instance, if a community chooses to address motor vehicle speeding issues, the Green Book provides guidelines for narrowing roadway lanes to help reduce vehicle speeds and pedestrian crossing distances.

While references to AASHTO, MUTCD,RDM, HCM and ITE trip and parking generation reports may remain mysterious, you should now have a better idea of why these manuals are relied upon. It is important to remember that these manuals are not design prescriptions. Flexibility exists in how design recommendations are applied and you shouldn’t be afraid to speak with your local engineer about designing projects that fit the transportation needs of all users.
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