Methods and Approaches for Sight Distance by Functional Classification and Context
engineers use various sight distance criteria, the criteria are generally based
on research along a limited-access freeway or a rural high-speed roadway. There is little to no research for sight
distance criteria in the urban or suburban context. For low to moderate speeds, sight distance
criteria are based on equations that were developed for higher-speed
environments and may not apply perfectly.
National Cooperative Highway Research Program (NCHRP)
15-47 states that the sight distance model is currently not a performance-based
model. For example, object height is a
set value based on a value from taillights, which has not been changed in
several decades. The values of
determining sight distance should be evaluated in order for design standards to
be more inclusive. Sight distance can
impact costs of reconstruction due to values that are being utilized for inputs
into the sight distances equations. Changes
in inputs will allow the model to be more flexible to different types of
functional classification and context and is consistent with project based
This research would examine the various sight
distance criteria along highways with functional classifications and contexts
that are not covered in previous research.
The research should examine stopping, decision, and intersection sight
distance as well as sight distance requirements for various modes such as
pedestrians, cyclists, and transit vehicles. The research should reflect the
risk of a crash into the analysis.
Various speeds, traffic volumes, the dynamics of moving vehicles, parked
vehicles, and crash data should be considered in the analysis.
of the research include:
· Develop sight distance criteria that is performance-based and assesses
· Provide engineers more flexibility in implementing sight distance
criteria in various contexts
· Balance the needs of safety with costs
The potential benefits are to get greater value from projects, potentially implement projects faster, and allocate funds properly to the needed safety issues based on performance and not nominal data.
The literature for sight distance
is well referenced in the 2011 Edition of the AASHTO Green Book (1). NCHRP Project 15-47 (2) discusses the future
research needs for sight distance.
Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets, Washington,
Once the project is complete, the goal will be
to incorporate the results into the AASHTO Green Book as well as other guides
that are geared for urban and suburban contexts.
As transportation funding is more and more limited, the urgency for this research gets greater and greater. Agencies do not have enough money to design roadway projects that meets every sight distance need. In some cases, sight distance criteria are applied even though the safety risk for the added design might be extremely low. There needs to be a better balance in making engineering decisions based on limited budgets. Funds can then be reallocated to address other safety issues that are based on performance.
There is a separate urgency to evaluate sight distance criteria for other modes. There is no criteria in the US for pedestrian or bicycle sight distance. Similarly, there are no criteria for buses, light rail, or other transit vehicles.
|Sponsoring Committee:||AKD10, Performance Effects on Geometric Design
|Research Period:||12 - 24 months|
|RNS Developer:||Gilbert Chlewicki, Jeff Shaw, Jason Hodges|
|Source Info:||Developed as part of the 2016 mid-year meeting of the TRB Committee on Geometric Design (AFB10), TRB Committee on Operational Effects of Geometrics (AHB65), and AASHTO Technical Committee on Geometric Design.|
|Index Terms:||Highway design, Sight distance, Vehicle dynamics, Traffic volume, Crash data, Risk assessment, |
Operations and Traffic Management