DEVELOPMENT AND TESTING OF A LOW-PROFILE BARRIER
Figure 1. Current design concept of the low-profile barrier (posts span 10 feet o.c.).
Background
In an effort to improve the aesthetics of Caltrans right of way, several Districts have requested to plant trees in the medians of low-speed highways. Trees with an expected mature size greater than 4 inches are considered fixed objects and must be removed or shielded. When trees or shrubs with multiple trunks or groups of small trees are near together, they also pose as a hazard because they can be considered as having the effect of a single tree with their combined cross-sectional areas (1). Otherwise, they must be at a minimum of 30 feet from the traveled way (2), which is usually not possible in urban environments. Currently there are no low-profile aesthetic barriers suitable for this purpose. Hence, many municipalities are unable to plant trees in context sensitive environments.
An effort is being made to develop a non-proprietary, permanent, low-profile, narrow barrier that can be used with or without soil backing on the non-traffic side. The barrier needs to pass test levels 2-10 and 2-11 under NCHRP Report 350 (4) guidelines. The barrier design concept is shown in Figure 1. The cross-sectional measurements are shown in Figure 2. Regarding aesthetics, the leading request is for openings in the barrier for a less monolithic and more see-through appearance. (Note: design concept only shows the above ground portion of barrier. Footing is currently in the conception stage.)
Figure 2. Cross-section of design concept (units are in inches).
The result of this project will benefit motorists both in safety and highway aesthetics for locations where the barrier is installed. It will allow municipalities to develop context sensitive environments by planting trees along state highways where they are currently prohibited. The project permit process that includes an aesthetic alternative may decrease project permit delays by local agencies. In addition, motorists will be protected from the potential hazard of colliding with trees too close to the traveled way, should they run off the road. The barrier will be designed according to federal requirements to redirect the vehicle safely without serious injuries to the occupants.
References
- Roadside Design Guide. American Association of State Highway & Transportation Officials. Washington, DC. 2002 edition (3rd edition): 4-15.
- Highway Design Manual. State of California Department of Transportation. Sacramento, CA. 6th Edition: 900-4.
- Traffic Manual. State of California Department of Transportation. Sacramento, CA. Revised Sept. 13, 2002: Section 7-02
- Report 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features. Transportation Research Board. Washington, DC. 1993.
Contact Information
Vue Her
Vue.Her@dot.ca.gov
Project Engineer
John Jewell
John.Jewell@dot.ca.gov
Senior Engineer
Bob Meline
Bob.Meline@dot.ca.gov
Senior Engineer