Drivers travelling between Radlett and Shenley are facing further disruption this week.
Unplanned repairs are being carried out by Affinity Water in Shenleybury, that started today (Tuesday).
Three-way traffic lights have been set up between Shenleybury Cottages and Farm Close.
The work could last until Thursday at the earliest.
The roadworks caused some delays during this evening's rush hour, particularly down Black Lion Hill towards London Colney.
The roadworks untimely because Shenleybury is currently being used as a diversion while Harper Lane is shut by the junction with Watling Street.
That closure, caused by a crash in late September that damaged a railway bridge, could last for the whole of October.
Read more: Harper Lane businesses slam 'horrendous' new bridge closure
Notable delays are very likely in the morning (Wednesday) in and out of Shenley and motorists are advised to leave extra time if travelling.
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