[Joe Staff]

Of course it's possible, and it requires more math and less scripting skill.

pawn Код:

new pSpeedLimit[MAX_PLAYERS]={-1,...};
new Float:gTempFloat[4];
public OnPlayerConnect(playerid)
{
  pSpeedLimit[playerid]=-1;
  return 1;
}
public OnPlayerCommandText(playerid,cmdtext[])
{
  if(!strcmp(cmdtext[1],"setlimit",true,8))
  {
    pSpeedLimit[playerid]=strval(cmdtext[10]);
    return SendClientMessage(playerid,0x99CC9900,"  Speed Limit Set");
  }
}
public OnPlayerUpdate(playerid)
{
  if(pSpeedLimit[playerid]>-1)
  {
    GetVehicleVelocity(GetPlayerVehicleID(playerid),gFloatTemp[0],gFloatTemp[1],gFloatTemp[2]);
    gFloatTemp[3]=(floatsqroot(gFloatTemp[0]*gFloatTemp[0]+gFloatTemp[1]*gFloatTemp[1]+gFloatTemp[2]*gFloatTemp[2])*100)/pSpeedLimit[playerid];
    if(gFloatTemp[3]>1)SetVehicleVelocity(GetPlayerVehicleID(playerid),gFloatTemp[0]/gFloatTemp[3],gFloatTemp[1]/gFloatTemp[3],gFloatTemp[2]/gFloatTemp[3]);
  }
  return 1;
}

Tested and works to 80% satisfaction
*issues: May cause lag at higher player bases and higher pings. Occasionally it doesn't work and player will have uncapped speeds until completely stopped (as far as I can tell)

I think maybe my speed calculation is incorrect, but it seems fairly accurate.