Closing the M4 bus lane doesn't fix the eastward bottleneck, which is the 2-lane Brentford Flyover. In fact, closing it will likely make congestion worse, because:
- it will require 2 lanes of traffic to merge at the flyover, which will causes more crashes (which cause delays, duh) as well as disrupting traffic flow;
- Buses will get caught up in the newly-created congestion in the 3-lane section. The increased bus journey time will make some people take to their cars, adding to the congestion
Here's an explanation of why the M4 bus lane works, improving journeys for all traffic.