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FHWA has noted a tested post spacing range for cable barrier intermittent post of 6.5 ft to 15 ft.  In the documents I have found they normally talk about larger post spacings may allow for a vehicle to underride or go through the cables but I have not found any comments about why there is a lower limit of 6.5 ft.  After talking to one of our cable barrier suppliers, they would not recommend using a post spacing less than 7 ft for their product because they fell the closer post spacings may lead to vehicle instability.  Is there any research that notes any concerns with vehicle instability in cable barrier systems with small post spacing, 6.5 ft or less?

Post Spacing: Cable barrier systems have been tested and accepted with post spacing ranging from 6.5 feet to 32.5 feet. In general, deflection distance is known to increase with longer spacing between posts. What is not known, but strongly suspected, is that longer post spacing may also affect the propensity for vehicles to penetrate the cable barrier, i.e., by underride or traveling between cables. The FHWA recommends that highway agencies specify the post spacing when cable barrier systems are bid. The conventional range for cable post spacing is 6.5 to 15 feet.

Above is from an FHWA Memorandum:  https://safety.fhwa.dot.gov/roadway_dept/countermeasures/reduce_crash_severity/policy_memo/memo072007/memo072007.pdf

There has been previous research regarding cable barrier crashese that found that the use of larger section cable posts or reduced cable posts spacing could potentially induce vehicle instability in small car impacts. We have seen related instabilities during testing of cable end terminal systems. The report with the crash data analysis is located below.

https://mwrsf.unl.edu/reportResult.php?reportId=335

Determining the allowable post spacing for a given barrier system would require consideration of several factors including the post section, posts spacing, and overall barrier configuration. As such, we would generally recommend following manufactuer guidance unless additional research is peformed. 

thanks