For iphone brothers and sisters (courtsey of rpcameron)
You must be using an Android device. On the iPhone, 5 quick presses of the side/power button (or long-pressing power+volume) will bring up the Power Off/SOS menu; any future attempt to unlock will require the passcode. (Either action can be down without any screen interaction, meaning that you can enable this feature silently as soon as you feel it necessary.)
(Also to note for iPhones: if you choose a 7 digit or longer passcode, the entry field does not indicate how long the passcode is; the same is true if you choose an alphanumeric passcode.)
(Extra safety for those in the US if you are in a car, after doing the above stash your phone in the console/glove box; if it is within a sealed compartment not on your person additional cause/warrant is required to gain access to the device.)