Preheat oven to 400 degrees. In a large, shallow roasting pan, combine carrots, turnips, squash, and garlic. Drizzle with olive oil and sprinkle with thyme, salt, and pepper, then toss until vegetables are coated with oil. Roast vegetables, uncovered, for 10 minutes. Reduce heat to 350 degrees and continue to cook, stirring once or twice with a metal spatula, until vegetables are tender, about 40 to 50 minutes. Season with additional salt and pepper before serving.