How to know when you need a DOT number: If your GVWR for your truck and trailer combined is over 10,000lbs or 10,001+ you need a DOT number. This has nothing to do with the actual amount of weight that you are carrying. If your truck and trailer actually weighs over 10k lbs you must be DOT certified but if your truck and trailer is even rated to carry that amount of weight you must be certified as well. Here are some examples. If you drive a 1/2 ton truck with a GVWR for the truck of 6000lbs (find this information on the drivers door) and pull a single axle trailer with a 3500lb axle you do NOT need a DOT number because your combined GVWR is only 9500 lbs. If you drive a 1/2 ton truck with the same GVWR of 6000lbs and you pull a tandem axles trailer with two 3500lb axles you MUST have a DOT number. Your combined GVWR is now 13k lbs. Even if you do not load your truck to over 10k lbs! It is about the gross vehicle weight "rating", not the actual amount of weight on the truck. This means that if you drive a 1 ton truck with a GVWR of 9500lbs if you pull a trailer with a GVWR of more than 500lbs you must have a DOT number. No exceptions. Just remember, it doesn't matter how much weight you are actually loaded with it is about how much weight your truck/trailer combo is rated for to determine if you need a DOT number. What if I load my trailer and truck with more weight than the combined GVWR to get away from a DOT number? So you want to load a 1/2 ton truck with a single axle trailer up to the hilt and pull around 15k pounds? Big problems! If you get pulled over by a State Trooper and they want to weigh you they have individual scales that will go under each tire to weigh in. If you weigh even one pound over your GVWR you will have big fines and they may shut you down right then and there! Even further more your truck has a Gross weight rating for each axle. If one of those axles are over weight you are in trouble. Make sure you know how much your rig weighs! What about my License Tags? Your license plates are a little bit of a different beast. In Kansas, a standard truck plate is marked with 12M. This means that the truck/trailer can not weigh more than 12k lbs. But, if you drive that 1 ton truck with a GVWR of 9500lbs and pull a tandem axle trailer with two 7,000lb axles you would presumably have combined GVWR of 16,500lbs. You need a DOT number, obviously but as long as the weight of your truck, trailer and all of the load and occupants does not exceed the 12k lbs that your truck tag is rated for you do not need a different License Tag. Remember this one thing about tags. The truck tag must cover the weight of the truck and the trailer combined. The trailer tag only has to cover the weight of the trailer. This is how it is in Kansas, anyway. Chime in if its different in your state. Again, if a trooper weighs you and you're over your Tag's ratings you're sunk as well. What else do you need to be DOT compliant? In Kansas in order to be DOT compliant there is a HUGE list of criteria that must be met. I was in charge of making sure a company's trucks and trailers were compliant 5 or so years ago and it was a full time job! Just some of the basics: Every axle on your truck and trailer that touches the ground must have brakes. Triple axles trailer? All three axles must have brakes (This is not in every state) Every trailer must be equipped with a break away cable box. Further more if you get stopped by a Trooper you must be able to stop your rig from 10mph with just your trailer brakes from the controller in your cab. And within so many feet. He will also pull your brake away box and when you pull forward every tire that touches the ground must lock up and drag. Not a single one may roll! You also have to have emergency triangles in the truck and a fire extinguisher "firmly" attached within reach of the driver. Not just rolling around behind the seat. It must be screwed to the floor! (This will give you a break on insurance, too, BTW) Every driver that wishes to drive a DOT numbered truck must go through a DOT physical at a Dr's office and carry a DOT card with him at all times. This physical gets completed every 4 yrs (I believe) If you have a DOT number it must be on your truck in a visible location AND your company's name must be visible and eligible. Then you get into the lights: Any truck or trailer that is over 8 ft wide (i think thats the number) must have identification lights. These are the three red lights in the middle of the rear of the trailer or on the back of dually trucks. This is so cars don't try to drive "thru" your tail lights thinking your trailer or truck is just two motorcycles. You must also have clearance lights at the widest points of your trailer and DOT approved reflective tape as necessary to comply. AND THAT'S JUST THE BEGINNING! Oh and make sure you loads are secure. If you drive an enclosed trailer you can't be picked on but if you pull an open trailer everything better be tied down. Unless you have 4 ft sides than you're ok (In Kansas). Say you have a fully open trailer and you have 80 bags of mulch spread across the flooring. You cannot just throw a tarp over it and go on with life. The trooper will not call that "secure" you must throw pallets or 2x4s and strap those things in place! But make sure you tarp your debris or you'll get fined for littering. Both from the Trooper at the State level and when the City learns about it they will send you a fine in the mail too. (FYI) When I worked for this company we also had to get certified by the City (Wichita, KS) to be a solid waste collector. This was for our clippings and what not. Had to pay a fee every year and get stickers on the trailer or truck we were putting our debris into. Also we had to have a bulk water transporter sticker through the city since our spray rigs were 300 gal tanks. Again, this is mostly from KS so make sure you check your local regulations. OH! And if you plan on doing work in another state to work you must have a DOT number for that state as well. This is why most big rigs have USDOT numbers which will cover them as they travel through other states. But if they stop to set up shop in a state they have to be DOT certified for that state as well. If you only do work in one state you only have to follow the laws at the state level. If you go to a different state you must comply with that states rules too. If you go all over the country you must follow the federal laws. If your DOT certified truck is also your daily driver you must be in DOT compliance at all times. Even if you're pulling your RV to the lake. If your wife drives your truck to get groceries, legally, she is supposed to be DOT certified as well. Whether she's pulling a trailer or not, as well. GOOD LUCK! and thanks for reading... chime in if you know of anything else.