Pothole on a Residential or Local Road?
If Southwark or a local authority managed road/track, then we’ve got responses by using https://www.fixmystreet.com/

Pothole on a Major Road?
Emergency street faults and dangerous potholes that need to be dealt with urgently can be reported by phoning 0343
222 1234 (option 6). The issue will be escalated to London Highways Alliance Contractors (LOHAC) to respond, fix the problem and provide information on the resolution, which is usually on the same day. (There will be a TfL website for this soon)

TfL maintenance issue?
Report street faults (including traffic light faults) using their https://tfl.gov.uk/info-for/urban-planning-and-construction/roadworks-and-street-fault. Select “Make a complaint” and Streets”

Systems like “fixmystreet” are not designed to report direct to TfL for issues on their redroutes, and only log and pass to Local Authorities

Local safety issue?
Contact local Ward Councillors. Use https://www.gov.uk/find-your-local-councillors if you can find a nearby post code (assume the road is not cross-border).

Some GLA members lobby TfL on a range road safety issues. If there is wider issue that the GLA have designed in a safety issue, or not taking reasonable action to mitigate known dangers, then they can help raise questions to the Mayor.

TfL “design” issue?
CUSTOMERSERVICES@TFL.GOV.UK
Very informative replies on a range of issues, and allows concerns to be addressed by the right TfL team. Responses are not quick.

New Design probem?
TfL “Road Safety Audit” defines the process they should be following post implementation. Stage 3: RSA should be completed within 3 months following completion of the scheme. Stage 4: Monitoring are not routinely undertaken on TfL internally promoted schemes. So may need to be asked.
http://content.tfl.gov.uk/tfl-road-safety-audit-procedure-may-2014-sqa-0170.pdf

In my experience, all-new layouts always take some bedding in as habitual behaviour is forced to change. It’s also possible that knee-jerk reactions to issues can cause more serious unintended consequences. It is therefore important not to make hasty changes to layouts after implementation.