Plan extra time during rush hour; roads leading to Heathrow, including the M4, often experience congestion

https://www.scribd.com/document/1074920514/Getting-to-Heathrow-from-Gatwick-Easiest-Inter-Airport-Transfers-178501

Plan extra time during rush hour; roads leading to Heathrow, including the M4, often experience congestion, especially around junctions and airport exits.