Hi Stefi If the traffic is 'at a standstill' it has stopped, usually, but not always, due to congestion or some other incident that has caused everything to stop. If there is a holdup, then it does not necessarily mean that the traffic has stopped (at a standstill), but will frequently mean simply that a journey is delayed or slower due to some reason or other, such as an accident or congestion. So, you can be held up (by a 'traffic jam') without actually stopping.

The difference is in usage. Here are the standard usages: "Traffic was AT A STANDSTILL." "There's A HOLD-UP on Route 128." "I was HELD UP by traffic." Here are other common idioms re: traffic: "Traffic is crawling." / "Traffic is moving at a crawl." "It's bumper-to-bumper traffic." "We were bumper to bumper for two hours over the bridge." "Route 128 is a parking lot."