Hertfordshire County Council has said it does not know what caused temporary traffic lights on the Oxhey Lane to cut out during rush hour yesterday evening.
Drivers were left to negotiate the junction on the 40mph arterial road with Little Oxhey Lane without guidance after the lights failed around 6.30pm.
The council said workmen were at the site by 8pm and the lights were up and running again by the early hours of this morning.
The junction is currently undergoing four-weeks of work to replace the lights, which are due to be complete on January 31.
The works are part of an ongoing programme of replacing traffic lights which are over 15-years-old.
At the end of the month another four-week schedule of work will start on the lights at the junction of Whippendell Road and Hagden Lane.
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