The new guidance means traffic should give way when pedestrians are crossing or waiting to cross at junctions.
Cyclists are advised to ride in the centre of lanes on quieter roads, in slower-moving traffic, and when approaching junctions, to make themselves as visible as possible.
A hierarchy of road users is also being introduced, meaning someone driving will have more responsibility to watch out for people cycling, walking, or riding a horse, and cyclists will have more responsibility to be aware of pedestrians.
The Highway Code contains advice and rules for people on Britain's roads.
Nine sections have been updated, with 50 rules added or amended.
The changes are advisory, so non-compliance will not result in a fine.
AA president Edmund King expressed concern at the potential impact of the guidance to give way to pedestrians at junctions.
He suggested that drivers are "likely to get hit by another vehicle from behind" if they stop on dual carriageways or fast-flowing A roads to let someone cross.
He also warned that pedestrians could be endangered if one vehicle gives way but another travelling in the opposite direction fails to stop.
"Drivers will have to make their own judgments on what they should do in the scenarios they find themselves in," Mr King told the PA news agency.
"However, if the judgments of the driver and the pedestrian are at odds on a very busy road, this could lead to problems."
Asked later about how the changes are going, Mr King said: "It is quite interesting because obviously it was introduced today and it is quite difficult to ascertain."
He also said it is "early days" and "not everybody knows about the changes".
But he added: "We have had no reports from our patrols or driving instructors of anything going drastically wrong.
"Many of these changes are already just common sense and courtesy."
Mr King described how he had personally been out on a bicycle, in a car and on foot on Saturday, saying: "On each occasion, the particular right of way did seem to work.
"The truck gave way to me and when I was on a cycle I gave way to pedestrians in the road."
An AA survey of more than 13,700 drivers carried out earlier this month indicated that 33% were unaware of the changes, including 4% who had "no intention" of looking at the details.
RAC head of roads policy Nicholas Lyes warned that the "substantial" changes carry a risk of sparking "angry clashes and, worse still, unnecessary collisions" unless all road users are aware of them.
He added: "Nobody wants to be on the right side of the Highway Code changes but in the back of an ambulance because of confusion on the part of a driver or any other road user."
A communications drive will be launched by the Department for Transport's road safety offshoot Think! in mid-February, with further activity later in the summer.
Roads minister Baroness Vere said this week that the updated Highway Code will make Britain's roads safer and encourage people to "respect and consider the needs of those around them".
Mayor of London Sadiq Khan said the new instructions for road users will "help us make London the best city in the world to walk and cycle".
Charity Cycling UK said the changes must be "communicated with simple, accurate and memorable messaging".