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I had a question come up on an interchange project. 
Due to soil remediation they were not able to install standards W beam
guardrail which would have allowed them to flare the installation.  They
plan to install a short section of concrete barrier and a crash cushion
instead.   The problem this causes is the tangent installation causes
a sight distance problem.  Would it be possible the flare the concrete
barrier at a 20:1 rate and install the crash cushion along that same
line?  We could flare only the concrete portion but that would require a
greater length of concrete rail to maintain the desired sight distance.

With respect to the situation below, we would not have an issue with flaring of the permanent concrete barrier as long as it falls within the guidance in the RDG for barrier flare rates based on the shy line and the design speed.

In terms of the crash cushion, we would likely recommend that the crash cushion be extended parallel to the roadway rather than in line with the flared concrete barrier due to potential concerns for impacting the crash cushion at angles and severities outside the range of its original testing. To the best of my knowledge, there has been little research or testing on crash cushions installed on flares. Minor flares have been allowed for terminals in many cases.

That said, I would recommend that you contact the crash cushion manufacturer directly in order to get their input. They likely have better insight than we do with regards to their systems in flared installations.