I assume you figured it out by now but you can do it by looping through the radio buttons to check which is selected.
Code:
<html>
	<head>
		<title>Radio Button Selection</title>
		<script language="javascript">
		function SelectFare() {
			chosen = ""
			len = document.frm.Vehicle.length

			for (i = 0; i <len; i++) {
				if (document.frm.Vehicle[i].checked) {
					chosen = document.frm.Vehicle[i].value
				}
			}
			alert(chosen) 
		}


		</script>
	</head>
	<body>
		<form name="frm">
			<input type="radio" name="Vehicle" value="Car_Single" onclick="SelectFare()"  />
			<input type="radio" name="Vehicle" value="Car_Return" onclick="SelectFare()" />
			<input type="radio" name="Vehicle" value="Estate_Single" onclick="SelectFare()" />
			<input type="radio" name="Vehicle" value="Estate_Return" onclick="SelectFare()" />
			<input type="radio" name="Vehicle" value="Exec_Single" onclick="SelectFare()" />
			<input type="radio" name="Vehicle" value="Exec_Return" onclick="SelectFare()" />
			<input type="radio" name="Vehicle" value="Carrier_Single" onclick="SelectFare()" />
			<input type="radio" name="Vehicle" value="Carrier_Return" onclick="SelectFare()" />
			<input type="radio" name="Vehicle" value="Bus_Single" onclick="SelectFare()" />
			<input type="radio" name="Vehicle" value="Bus_Return" onclick="SelectFare()" />
		</form>
	</body>
</html>