The die is known as the speed die
@Cascabel's answer has the rules from the championship edition for this die.
Mega Edition has different rules (especially surrounding the bus symbol). The mega edition rules are compatible with a regular monopoly set if you add 16 bus tickets. To make your own, you need 13 regular ones and 3 that say "all other tickets expire"; the two types of bus tickets should be indistinguishable when face-down. You could just use playing cards (taking 13 diamonds to be regular bus tickets and 3 spades to be the "all other tickets expire" tickets). Those rules are included below.
Regardless of which edition you are using, the speed die does make the game shorter, both in making properties get bought faster then in causing players to go bankrupt faster. It does also somewhat radically change late-game strategy due to the behavior of the Mr. Monopoly symbol once all properties are bought (move to the next property where you owe rent). This rule allows a player to augment the likelyhood of other players landing on a heavily developed property by buying a contiguous block of the preceding properties and mortgaging them all (thus making the developed one the next place another player would owe rent for a large chunk of the board). This also increases the value of railroads and utilities as potential "spoilers" in these runs of mortgaged properties.
Mega Edition Rules:
THE DICE & HOW TO MOVE
On your turn, roll all THREE dice: the two white dice plus the new
• If pips appear on all three dice, simply move the total of all
• If you roll a Mr. Monopoly, you get a bonus move. First, move
according to the white dice, do what you would normally do on that
space; then move again to the next unowned property, which you may now
buy. If you roll a Mr. Monopoly after all of the properties are
owned, make a normal move according to the white dice, then move ahead
to the first property on which you need to pay rent. If all other
players properties are mortgaged, stay where you are.Note: An opponent
can’t build or trade until your Mr. Monopoly move is completed.
• If you roll the Bus, first make a normal move according to the white
dice, then decide to either take a BUS TICKET for later use(usually a
good idea if any tickets are left in the deck) or move again to the
nearest Chance/Community Chest space in front of your token
DOUBLES & TRIPLES
Refer only to the white dice when determining doubles. If
doubles entitle you to roll again, you may use a Bus Ticket in place
of rolling the dice. While in JAIL, roll only the two white dice (if
you pay beforehand to get out, roll all three dice). If you roll
TRIPLE 1’s, 2’s, or 3’s, move ahead to ANY space on the board. Do not
roll again. You do not go to Jail if you’ve rolled doubles twice
before rolling triples.
On any turn, you may choose to use a Bus Ticket (if you have any)
instead of rolling the dice. If so, move forward to any space on
the same side of the board. (Corners are the final space on
each side, as well as the first space of the next side.)
After using a Bus Ticket, give the Ticket to the Banker,it is out of
play for the remainder of the game. (There are 16 tickets in all.
When they are gone, they’re gone!) Watch out! When an “All Tickets
Expire” card is drawn, all other Tickets in the hands of the players
are lost to the bank (they’ve expired). If a Bus Ticket or Mr.
Monopoly roll causes you to pass GO, collect $200 as usual. Note:
All Bus Tickets should be revealed as collected and kept face up.