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Methods and Approaches for Sight Distance by Functional Classification and Context


When 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 project development.

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.


The objectives of the research include:

· Develop sight distance criteria that is performance-based and assesses risk

· 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.

Related Research:

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.


  1. American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets, Washington, DC, 2011.

  2. NCHRP 15-47, http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=3415


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
Research Priority:Medium
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.
Date Posted:10/26/2016
Date Modified:11/14/2016
Index Terms:Highway design, Sight distance, Vehicle dynamics, Traffic volume, Crash data, Risk assessment,
Cosponsoring Committees: 
Operations and Traffic Management

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