Actually you can walk along the causeway I think...but that's for the past(Not sure about now). I've seen many gave up on queuing 170/160 and ended up walking from Wdl CIQ to JB CIQ. In the past, waiting for 170 can be long.
There's also a gate there. But I don't think there would be anyone interested to walk in now, given the fact that 160/170/950 can reach the JB fast and the frequency is still acceptable(on average).