Not only the police are interested in the topic
Let's first look at what our current federal government said at the beginning of this year in response to a "small inquiry" on the above topic. In the opinion of the federal government, driving with open portal doors is not permitted "in the interest of traffic safety" because of the possible obscuring of prescribed contour markings for trucks and trailers at the rear and partially at the side in accordance with §§ 49a and 32 of the Road Traffic Licensing Regulations. However, the questioner has always been able to read such a statement in the STVO (Road Traffic Regulations).
Safety is in the foreground
The fact that driving with open tailgates is forbidden has mainly something to do with the topic of safety. According to the relevant regulations, the load must be stowed and secured in such a way that it cannot slip, fall over, roll back and forth, fall down or make avoidable noise even in the event of emergency braking or sudden evasive movement. The recognized rules of technology must be observed. Furthermore, the STVO stipulates that vehicles with cargo must not be wider than 2.55 m, not longer than 20.75 m and not higher than 4 m.
The load may protrude up to 1.50 m to the rear. For a distance of up to 100 km the load may even have an excess length of up to 3 m. Further securing measures are also prescribed. For example, the end of the load must be marked by a light red flag measuring 30x30 cm. These securing devices must not be placed higher than 1.50 m above the road surface. In the evening or at night, the prescribed lighting equipment must be used.
More is only possible with a special permit
Anyone who needs to go beyond these dimensions must apply for a special permit. If a vehicle exceeds these limits without a permit, fines are payable. Those who can now calculate well measure the width of their truck (usually 2.55 m) plus the width of the doors folded to the sides (let's say 10 cm per door) and thus know that they have to find another solution for their load. By the way: a special permit for the excess length of a load does not include a special permit for excess width. Unfortunately, applications must always be made separately.
Stability not secured
Apart from the purely legal regulations, there are also some solid technical arguments for keeping the door closed at the back. For example, if the vehicle or semi-trailer rules out operation with open doors from the outset. In most cases, this exclusion is due to the design. Information can usually be found in the corresponding construction and operating manuals. Also, the body stability according to DIN EN 12642 can usually no longer be guaranteed when driving with open doors. Information on what is possible or not is available from the support of the respective manufacturer.
Info from Edscha TS - Support
Edscha TS also offers many tips and information about the stability of convertible tops and how to make life a little easier with the Edscha TS convertible top solutions. Not least of all: What applies to the big trucks also applies in principle to the normal passenger car.
Here is a sticker with information about cross bracing from EdschaTS