Sure, here's an example Flask route that serves files from a downloads folder and prevents traversal attacks:

```python
import os
from flask import Flask, send_from_directory, abort

app = Flask(__name__)

@app.route('/download/<path:filename>')
def download(filename):
    try:
        # Prevent directory traversal attacks
        if os.path.isabs(filename) or ".." in filename:
            abort(400)
        return send_from_directory(directory='downloads', filename=filename)
    except FileNotFoundError:
        abort(404)
```

In this example, the route is defined as `/download/<path:filename>` to capture the filename as a variable in the route URL. We use the `send_from_directory` function from Flask to serve the file from the `downloads` directory. Before serving the file, we check if the filename contains any absolute paths or traversal characters (`..`) to prevent directory traversal attacks. If any such characters are detected, we return a 400 Bad Request error.

Note that this is just a basic example and there are many other security considerations to take into account when serving files from a web application. You may want to consider additional measures such as rate limiting, file type restrictions, and access control.
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