Toss coleslaw mix, radishes and scallions in a large bowl. Whisk vinegar, oil, sesame seeds, lime juice, crushed red pepper and salt in a small bowl. Refrigerate the slaw mixture and dressing separately until needed.
Up to four hours before serving, pour the dressing over the slaw and toss to coat. Transfer to an airtight container and chill until just before serving (see Tip).
Tip: If you won't be serving the slaw within 4 hours of leaving home, tote the cabbage mixture and dressing in separate containers in the cooler. Combine and toss to coat before serving.