Maintenance is required for the signals, at the Chapel Road junction with Richmond Road, as the existing signals, electric cables, and ducting are all over 30 years old and must be changed.
Work is due to start on Monday, April 20.
Two pedestrian islands will be replaced by one – to make it easier for people to walk to and from the town centre – and tactile paving to help visually impaired pedestrians know where to cross is also being installed.
It is expected to take eight weeks to complete, subject to weather conditions.
Work will be carried out from Monday to Friday, between 7am and 5.30pm, and occasionally on Saturdays.
Temporary three-way traffic lights will be in operation throughout the duration of the works.
West Sussex County Council’s cabinet member for highways and transport, John O’Brien, said: “These works at the Chapel Road junction have been needed for some time.
“Now we have the funding in place we can get on with making the improvements.
“We apologise for any inconvenience this may cause motorists – please allow for extra journey time or consider an alternative route.”